Navigator 4.5 user manual
|Celestial Navigation links
- New to celestial navigation? To understand
how it works, go to the Fundamentals
- Other Navigator screen
- Navigator software Homepage
Both Navigator and ChartMaker programs have access
windows. These are the main menus of these applications. From
the access window you choose the module you want to work with.
For Navigator you have the following options (buttons):
- Chart Navigation
- Celestial Navigation
- Star Finder
- Make vector chart
- Import chart image
Take some time to read the License Agreement and disclaimer (click
"important information"). Closing the access window
will terminate the program.
The goal of the celestial navigation is to find the astronomical
position, the position of the boat. In this section we
will see how this can be done with the help of Navigator software.
As we have seen in the fundamentals,
crossings of two or more Lines of Position, taken for two or more
celestial objects, define this position.
But before you start taking altitudes of celestial objects, you
must be able to find them with the sextant. Trying to find a star
with the sextant on a rocking boat is not easy The eyepiece has
a relatively small angle of view and the sight is twisted by the
One technique to find a star is to turn the sextant upside down,
point it to the star, and bring the horizon by adjusting the arm.
Better yet is to know the approximate altitudes and azimuths
of the stars you are going to observe. This is known as preparing
Navigators wake up early. They do so to take advantage of the
two times of the day when the sky is in best condition for celestial
navigation: the twilights. In the civil twilights - times
when the sun is 6░ below the horizon - it's dark enough to see
the stars and planets, and light enough to see the horizon. This
happens before sunrise and after sunset.
The first step of the sky preparation is to determine the time
of the twilights.
To calculate the times of twilights do:
- Select the Sun in the celestial objects listbox.
- Set the date of the observation. Since you are probably going
to prepare the sun in the night before, set the date to the
- Set the assumed position (Latitude and Longitude). It's the
position you think you are going to be in the time of the observations.
- Select the tab "Other calculations"
- Press "Object data" button.
The last two lines show the time of twilights. Like this:
dawn civil twilight: 9:13 GMT
Set civil twilight: 20:46 GMT
Now that we know the twilight times, you can select the celestial
objects you are going to observe. At any time, Navigator
gives you more than 33 celestial objects to choose from. Of course
you will only need 4 or 5. To select stars and planets you will
observe, follow these guidelines:
- Select stars and planets that you are most familiar with.
- Select the brighter objects. Planets are easier to spot, because
they are very bright. Some stars are also very bright and easy
to find. Some constellations have distinct look and are easier
- Select objects with altitudes between 30░ and 60░. Less than
that you result in greater atmospheric refraction error, which
is not easy to correct (because it depends changing on atmospheric
conditions). And altitudes higher than 60░ are more difficult
- Do not select stars that have similar azimuths or that are
in opposition. The resultant Lines of Position will be nearly
parallel, which is undesirable.
To prepare the sky do:
- Set date and twilight time of your next observation.
- Set the assumed position (Latitude and Longitude).
- Select the tab "Visible stars".
- Press the "Calculate" button.
Now choose the stars in the spreadsheet or chart. To see the
name of star in the chart, click the mouse over it. The name will
show in a "hint label". Or select the star in the spreadsheet.
A circle will show around the correspondent star in the chart.
If a printer is available, Navigator can print a convenient
sky preparation (one page), with visible objects table and polar
When unsure about how to enter a value (date/time format,
number format or number unit), place the mouse cursor over
the input box. A hint will show with the field name, input
format and/or example.
Now lets take the actual measurements. Take the following items
to the deck of the boat:
4) Paper with your sky preparation. Attach the paper to a board,
so it's easy to take notes and your work will not be carried away
by the wind.
Try to establish a routine to handle these items. You will be
observing two numbers (altitude and time) at once, possibly on
a rocking boat, so don't let these items make things difficult
in the critical time. You might want to use preprinted tables
to organize your data, like the one below. Save them as the documentation
of your work.
|Astronomical position for ____ LOPs
to open this table in a new window, to print some copies
When taking an observation, set your sextant to
the expected altitude and point it to the expected azimuth (from
your "preparation"), using a hand compass. The celestial
body will probably show in your view.
- Adjust the sextant to the correct instrumental altitude. Write
name, time and altitude of the observed celestial object.
- It's good practice to adjust the sextant micrometer drum
always in the same direction. For example, put the star
below the horizon and then bring it up by turning the drum
in the same direction in all observations. If you go past,
repeat the operation from the start. Do the same for the
Index Error measurement. The sextant will give different
readings, depending on the direction you adjust the drum.
Using the same direction for both altitudes and index error
measurements cancels this problem.
- After adjusting the sextant's drum, read the watch first,
because it's changing fast. Write the time. Then write the
Before and after taking altitudes, measure the
- Set the altitude to 0░00' and point to the horizon.
- Adjust the drum until both sides of the horizon are level.
- Read the Index Error and write it.
Back to the navigation table, run Navigator.
- Enter the index error.
- Enter Height of the eye (Dip). In version 3.0+, you can choose
the Dip units (meters, feet or minutes) .
- Enter watch error, in seconds. See keeping the
time for more information on time keeping methods.
Note on the
index error: In versions prior to 2.5, the IE was
typed with the plus signal when the index error was inside
the drum scale. This number was then subtracted from sextant
reading by the program.
Many users complained that this input
convention was against the common practice in celestial
navigation. They were used to work with the Index Correction
(IC), with signal - when the IE was inside the
After many messages, I agreed. Starting
with version 2.5 and up, I changed this. So, if you are
upgrading, make sure you use the correct input convention,
as illustrated on the right.
|Navigator 2.5 and
|| type IC
(navigator 2.0 and prior)
Clear all previous LOPs:
- Select the tab "Astronomical Position".
- Press the toilet button, to clear all LOPs.
- Now select the "Line of Position" tab.
- Set the assumed position (Latitude and Longitude).
Now enter the measurements, one by one. For each celestial object,
- Select the celestial object from the listbox.
- Enter time of the observation.
- You can use local time or GMT time edit boxes. In this
case, automatic conversion to GMT is done.
- If using local time, make sure the time zone and watch
error edit boxes are correctly set.
- Enter altitude of the celestial body.
- Press the "Calculate" button.
The result will be something like this:
LOP for Sun
05/04/2001 13:43:54 GMT
Ass.Pos. Lat:23░40.0'S Lon:40░30.0'W
Inst. Altitude: 56░24.5'
Altitude of lower limb
Altitude corrections ------------
Par: 0.0' Refr:-0.6' SD:16.0'
Total Altitude Correction:10.3'
Corrected Inst Altitude: 56░34.8'
Object Positional data -----
Calculated Altitude: 56░37.8'
Intercept: -3.0 NM (away)
The two last lines have the results (Delta and Azimuth).
If you feel the result is consistent with the expected , press
the button "Save LOP". This will save this Line of Position
for the calculation of the Astronomical Position (Fix), which
will be done after you calculate all LOPs.
After calculating and saving all LOPs, go for the astronomical
- Select the tab "Astronomical Position". The LOPs
you have just saved will be in the spreadsheet.
- Press the "Calculate" button, to calculate the astronomical
position. Your astronomical position will show. Please note
that, in order to calculate the astronomical position, you have
to have two or more LOPs. Having more lines is advisable, because
errors in one observation will show more easily. A good number
is 4 LOPs. Also, as we will see, sometimes we are going to discard
some of them.
To achieve good results in celestial navigation, you need to
be methodic. As you can see, there are many steps, and a mistake
in one of them will only show in the end, if at all.
Enumerate the tasks you are doing - or are going to do - and
read the measurements loud before taking note (navigators are
said to speak to themselves). Make your notes in an organized
table, one row for each celestial object. In the header of the
table, write date, assumed position, time of twilight and index
error. Have the sky preparation ready before going to the deck.
But even with all the care, some errors will eventually show.
Wrong time or altitude (the so called 60 mile error). Bad star
identification. Even wrong date. The important thing is to detect
mistakes, and drop the LOPs with problem.
Having good dead reckoning navigation helps a lot. It's also
a good idea to take a look in the chart showing the Lines of Position.
If one of them seems out of the flock, you may deselect it and
recalculate de position. This is why it's good to have more lines.
Another problem is to have two or more LOPs that are nearly parallel.
They will probably cross very far from the correct position, even
if they are close together. Navigator accounts for this situation
by giving a small weight to crossings forming small angles. But
it's better to deselect one of them and recalculate the position.
LOPs 3 and 4 nearly parallel. Select only one
In the figure above, we can see that LOP 3 and 4 are nearly parallel.
Deselecting one of them would improve the resulting calculated
Observing the Sun
During the day, you can observe the Sun and the Moon. If you
can see both at the same time, and they are in suitable positions
for observation, you can calculate a fix, using the two lines
of position. The procedure is the same described above for stars
The only difference is that the Sun and Moon have appreciable
diameters (about 32'). When measuring the altitude of Sun and
Moon, align the lowest part of the body with the horizon. This
is known as the lower limb. Navigator will correct for the semi-diameter
You can also use the upper limb. In this case, uncheck the "Use
lower limb" checkbox.
The altitude of the same celestial object in two different times
may be used to find the position. For example, you can take two
Lines of Position for the Sun, one in the morning and one in the
afternoon. Because your boat is moving, you will have to transport
the first line to the second line time. The position obtained
with this method is called running fix.
Navigator (version 4.0 and up) can be set to transport LOPs automatically,
when calculating the astronomical position. This is done by moving
the assumed position. The LOPs chart shows the original LOP (blue)
and the transported (red).
Take a look at the Auto Transport frame n the image to
the left. This is where LOP transport is setup.
- Choose one of the following transport modes.
- Don't transport
- To latest LOP time
- To LOPs mean time
- To Selected Time (i.e. specified in the top time
- Click 'Edit boat C&S' (course and speed) and update
boat movement data.
- Click 'Calculate' to calculate the astronomical position.
When the Sun crosses our meridian, its azimuth is either 0░ or
180░ (North or South). This means that a Line of Position (LOP)
for the Sun, taken at this time, will have constant latitude.
This event is called transit or meridian passage. For the Sun,
it happens around local noon (+/- 20 minutes).
The navigator can take advantage of this event to check the latitude.
The longitude can also be calculated, although with smaller precision.
This is what you must do:
- Start taking observations of the Sun (time and altitude) about
25 minutes before the expected transit time. In Navigator,
select the Sun and use the command Object Data to estimate
the transit time for the Sun in your assumed position. Take
a couple altitude observations (p.e. 5 minutes apart) until
15 minutes before transit.
- At transit time, observe the highest altitude the Sun reaches.
This is known as culmination altitude. It´s easy to measure,
since the Sun will appear to hang with constant altitude while
passing your meridian. After that, it will start to go down.
It's important to understand that the Latitude result is related
to the maximum altitude the Sun reaches, and the Longitude to
the exact time of the passage. So, the latitude can be safely
determined even if you don´t have a reliable watch.
While the method just described is OK, is does not account for
two factors: Sun declination change and boat position change.
These two factors can affect the longitude result.
The Sun declination is always changing. It changes faster on
equinoxes (spring and fall) and slower in solstices (summer and
winter). So, the Sun 30 minutes after the passage is not in the
same place (in the celestial sphere) as 30 minutes before the
The boat movement during this period can also be of significance,
particularly if the boat is fast and is moving along the meridian.
These two changes affect the actual meridian passage time. In
this case, the average time between two equal altitude observations
(before and after transit) is not the meridian passage time, but
rather the culmination time. A correction must be applied to find
the right passage time (and the Longitude).
To perform this calculation on the Navigator, select the tab
"Other calculations" and click the
"Sun Meridian Passage by Equal Altitudes" button. The
form below will show.
Enter maximum altitude (w/o corrections), IE, Dip, GMT date,
GMT time of altitude 1 (before transit, w/o watch correction),
GMT time of altitude 2 (after transit), watch error, boat speed,
boat course and assumed position. The GMT times 1 and 2 are the
ones of equal altitude observations (the actual value of the altitude
is not used in the calculation, but remember to write it down,
because you will have to use the sextant to measure the maximum
altitude between the two observations).
The correction i is the difference, in seconds, between
the culmination time and the transit time. It can be as much as
a minute, or 15' in the longitude.
Please note that there are a couple conditions to use
- Sun altitude must be at least 65░.
- The Sun's azimuth must be at least +/-20░ on equal altitude
- The equal altitude observations must be up to 40 minutes -
before and after - transit time.
This method is particularly useful near the Equator.
If there are clouds in the sky, it's recommended that you
take several observations before transit. If you take only
one, the Sun may become covered in the critical time after
transit. By having many observations, you increase the chance
of having one usable observation pair.
Source: Navegação Astronômica
If you live in a city far from the sea, you can't take altitudes
of celestial bodies with a marine sextant, because you can't see
the sea horizon. One way to work around this problem is to use
an artificial horizon. The artificial horizon can easily
be made with a plate filled with liquid. Water will do, but oil
is better. The surface of a pool can also be used, if there is
no wind or waves (the water surface must be completely flat).
To take the altitude with the artificial horizon, point
the sextant towards the artificial horizon and make reflected
image of the celestial body coincide with the direct image.
The angle you read is twice the altitude of the body, as
illustrated in the figure below.
Also read the index error.
Navigator automatically corrects for the use of artificial horizons:
- The program divides instrumental altitude by two. Enter sextant
- The Dip and semi-diameter corrections are set to zero.
- The index error is also divided by two.
Navigator (registered version 2.5 or latter) has a Nautical Almanac
page generator/printer. These pages are not exactly the same as
real almanac pages, but they contain most of the information needed
to do celestial calculations in the traditional way, without the
Navigator generates the so-called "daily pages" (the
ones with 3-day celestial data for planets, stars, Sun, Aries
and Moon). The yellow ("increments") pages are not generated
because they don't change from year to year. You can use the yellow
pages of an old almanac or do the interpolations with a pocket
I choose to make the Navigator daily pages as similar as possible
to actual almanac pages. But there are differences:
- Did not include the latitude dependent tables (Twilights,
Sunrise, Moonrise, Sunset, Moonset).
- Did not include the Aries meridian passage time. This number
is used to calculate the meridian passage of stars, and is seldom
- Did not include the Sun's Equation of Time and meridian passage
table. You may use an old almanac for the Sun's meridian passage
calculations, as these tables are almost unchanged from year
to year. Just use the table of the same day.
- Did not include the Moon's meridian passage and age table.
- Did not include the planets' SHA and meridian passage table.
- Added three stars not included in most Nautical Almanacs
I plan to include some of these numbers in future versions. Feedback
from users about what features are most important is welcome.
If you compare the Navigator's almanac pages with nautical almanac
pages, you will note small differences in the numbers. These are
caused by different celestial calculation methods and should not
be bigger than 0.5'. This error is small when compared with other
imprecisions that affect celestial navigation, and will not impact
your position significantly.
Printing the almanac pages
To print almanac daily pages, do:
- Go to the celestial navigation window.
- Select the "Other calculations" tab.
- Press the "almanac pages" button. The "Almanac
Pages" window will show.
- Set the initial date for the 3-day page.
- Select "Aries and planets" (left side page).
- Press "Build page" and "Print".
- Select "Sun, Moon and Stars" (right side page).
- Press "Build page" and "Print".
- Click in the "Arrow" button to advance 3 days and
repeat operations from step 5). Proceed until you have printed
all the pages for the desired period.
In order to print your almanac pages, you have to use a non-proportional
font (a font with fixed pitch). The default font is Courier New,
size 7. Pages printed with this font will use a single sheet of
paper (size A4). I know this is a small font, but using a larger
one will result in two paper sheets for each page. You may experiment
with other non-proportional fonts. True type fonts are better,
because they can be resized to any small size.
Check the online web service, open to the public. Click to visit
Star Finder (version 3.0+) was completely redesigned. To accommodate
the new set of features, this chart of the visible sky was
moved to its own window (in previous versions, it was part
of the celestial navigation window).
lines and names make it easy to identify constellations
and celestial objects.
- Ecliptic plot
shows the path of the Sun, Moon and planets.
objects grid can be sorted by columns (ascending and descending
order) clicking the column header.
- The spreadsheet
now includes object magnitudes (note: many celestial objects
have changing magnitudes. These values are fixed, to be
used only as rough estimate).
- Mouse cursor
shows altitude and declination when moving. When pointing
an object, its name, altitude, azimuth, declination and
right ascension are shown. Values cursor is transparent,
so you don't loose the big picture.
- Prints the
object list and chart. The printed chart uses the printer
- Time zone
is now imported from the operating system, accounting
for Day Light Savings time.
background color indicates the light conditions (day=blue,
night=black and twilight=navy).
Clipboard support allows to cut-and-paste the chart
to other applications.
- Overhead view
option, so you can look up to the sky and the chart simultaneously.
In this representation, E and W are flipped and the chart
is to be viewed upside down
(see figure to the left)
- Options tab
allows easy and interactive chart setup (see figure to
- Boat course
plot, to easy printed chart orientation.
- Star finder chart and table now print in the same sheet
of paper, much more convenient.
Using the Star Finder is easy:
- Set your position (Lat/Lon)
- Set the GMT date and time. If you opened the star finder
by clicking the button in the celestial navigation window,
these values are automatically set.
- Set the boat course (optional)
- Click the Calculate button
You can also animate the sky, specifying time increments
and calculation frequency. This can produce very interesting
animations, like how the sky changes from hour to hour,
day to day and year to year.
And it can also be set to show the current sky.
The Navigator chart module is relatively new (started
in version 2.5), but has received important additions in version
4.0. It's now much more useful with the addition of raster chart
Instead of supporting the existing electronic chart formats,
I choose to provide a set of tools to allow the user to import
existing paper chart images into the program. This 'do-it yourself'
approach gives the user maximum flexibility.
All Navigator files are in text format and their structures
are easy to understand, to allow integration with other applications
and Internet file sharing. The table below enumerates the Navigator
data file types.
||Navigator vector chart file
||Navigator desktop file
||Navigator chart image description (raster
The Navigator chart engine can show two kinds of charts:
vector charts and raster charts.
Vector charts - In this type of chart, islands, continents,
routes and tracks are represented by polygons and lines, defined
by a collection of points (Lat/Lon pairs). This kind of chart
can be easily zoomed in and out, and is very fast to render. Navigator
uses its own vector chart file format. No other chart file format
is supported at this time. These files can be produced, from scanned
charts, using the ChartMaker program.
Raster charts - These charts are images (normally scans
of paper charts). Navigator can use raster images in formats JPG,
GIF or BMP. These are popular file formats in the
Internet, and many charts can be found for download over the Net.
All raster chart image files must first be imported - using ChartMaker
program - before they can be used in the Navigator. This
step is necessary to describe the chart image scale (i.e. how
pixels in the image map to the real world).
Both vector and raster charts can be produced or imported using
the ChartMaker program, which is described
in the next chapter.
In the image above you can see two raster chart images visible
in a desktop, on a zoom animation. Before opening it in the Navigator,
a raster chart image it must be imported. This is done only once
for each image, with the ChartMaker
program. The step creates a .CID file that is associated with
the chart image (GIF or JPG). This file contains:
- Name of the associated image file (GIF, JPG or BMP)
- File description
- Reference points (which define how the image pixels map to the
- Image MD5 digital signature (to prevent accidental changes in
the chart image)
- Chart limits (outside rectangle)
Once the CID file is created, it can be added to a desktop. Click
the chart image open button and select the CID file. Many chart
images can be added to a given desktop. Images can be either visible
or not. To play with chart visibility, use the small checkbox
in the chart images listbox.
Tip: Since images are large in size, making many
visible at the same time would consume a lot of memory.
Making only the charts you need visible reduces
the amount of memory used by the program. A memory of
at least 32MB is recommended to use raster charts.
The image resampling process also consumes a lot of CPU
power. To reduce the bumpy behavior while zooming or scrolling,
these calculations are implemented in different threads
of program execution . That's why it takes some time for
the raster charts to show after you zoom or scroll.
You can also play with raster image opacities,
making them partially transparent. This allows you to see how
two charts overlap each other or compare them with the vector
|Tip: Partially opaque (i.e. transparent)
chart images consume CPU power - a 100% opaque chart will
display a lot faster.
The chart images rendering order can also be changed, using the
up and down arrow buttons in the right. The rendering order is:
1- Background (sea color)
2- Vector chart polygons
3- Chart images (top-down order in the listbox)
4- Routes, tracks and marks
|Tip: To transfer a CID file to another
computer (or to share it over the Internet) , you also need
to transfer the associated image file (GIF or JPG). Since
the CID file contains the image file digital signature,
there is no risk of accidentally changing the image while
Navigator Desktops containing raster chart images
can be printed. You can use this to integrate different charts
together in a single and compact printout. Quality is even better
than the computer screen, because printers have a much higher
pixel resolution than monitors (monitors typically 75 DPI x
printers 300 or 600 DPI ).
This quality comes at a cost. Ressampled
chart images may turn out big, requiring a lot of CPU cycles
and memory to calculate. So be patient and make sure your computer
has at least 64MB of memory. If you have a laser printer, limit
the print resolution to 600 DPI. Don´t use 1200 DPI, because
this would result in really big ressampled image and would probably
hang your computer.
I´m working to make sure that the printing
process is smooth. You may find problems in extreme situations
(printing large chart images with high resolution printers and
little memory available).
Partially opaque (transparent) images are not
supported by printers. When printing, all chart images will
be rendered opaque.
The Navigator chart viewer has the following tools:
Zoom - Lets you zoom in and out the chart, from
the whole world to a very small scale. To zoom in, press
the left mouse button in the point you want to focus (screen
center). To zoom out, use the right mouse button or click
the left mouse button with Shift key pressed.
Measure - Click a position
and drag the mouse. The caption will show two numbers: COG
COG is Course Over Ground. It's the true direction
of the line. Range is the distance between the end points,
in nautical miles.
Both COG and Range are calculated using Lines
of Great Circle (LGC). This means that they are accurate,
even for large distances.
Navigator's vector chart interface
Route - This tool is used
to draw routes. A route is a set of points in the surface
of the earth, with optional associated text. Routes can
have any number of points or waypoints. You can draw as
many routes as you wish. Routes can be stored in Navigator
Desktop files (.NAV files). You can make some routes invisible
and concentrate on your current route. You can also edit
routes, change their colors and names.
At anytime you can check the COG and Range between
two points of a route, and the total length of a route. To edit
a route, click the checkbox "Edit routes/tracks". The
route editor will show. You can change all attributes of a route
(Color, Name, and Visible). You can also edit coordinates of points,
by double-clicking the point in the spreadsheet. To add a point
to an existing route (or track), click the route tool. In the
menu, select Routes Add Point to route. Choose the route.
Add one or more points. Right click to save points and end addition.
To create a new route, click the route tool and
click the route points. Right click end route. When prompted,
enter route name.
Tracks - This tool is used to draw
tracks. Tracks are like routes, except that each track point has
an associated date/time value. They are used to log the positions
on a trip. Like routes, they can have any number of points. Tracks
can be stored in Navigator Desktop files (.NAV files). You can
make tracks invisible to concentrate on your current track. You
can also edit tracks, change their colors and names.
To edit a track, click the checkbox "Edit routes/tracks".
The track editor will show. You can change all attributes of a
route (Color, Name, and Visible). You can edit point data, by
double-clicking the point in the spreadsheet.
To create a new track, select the track tool and
click the first route point (left button). If you want, add more
points with left clicks. When done adding points, right clicks
the chart. Enter track name.
To add a point to an existing track, click the route
tool. In the menu, select Tracks Add Point to route. Choose the
track. Add one or more points. Right click to save points and
Marks- You can also add marks to the desktop.
Select the mark tool and click one point, specify optional text
for the mark and choose a mark icon.
Pointer- Use the pointer tool to point chart
objects (islands, tracks, marks etc). If a name is associated
with the object, a text will appear near the cursor.
After loading a vector chart and/or raster charts; changing routes
or tracks, you can save all to a desktop file. A Navigator
desktop file contains the state of the chart viewer, including:
- One Vector charts.
- Raster charts (i.e. chart images)
Desktop files have the extension .NAV, and are in text format.
Navigator has GPS interface. This interface accepts NMEA (National
Marine Electronics Association) standard GPS messages. Two kinds
of NMEA messages are accepted:
RMC - Transit Specific Navigation information message-
This is the recommended (default) accepted message, because it
has date and course information. RMC messages give Latitude, Longitude,
date, time, course and speed.
GLL - Geographic Position Latitude/Longitude message
- Select this one if your GPS does not support RMC messages.
GLL messages have only Latitude, Longitude and time (no date).
|Note: The NMEA interface, available
in most GPS devices, uses a RS422 hardware interface.
This is not the same as the RS232c serial interface, available
in PCs. While the 422 interface uses +12/-12V electric
signals, the RS232 uses 0/5V. But since the PC uses the
level of about 3V to distinguish between 0s and 1s, the
connection works fine in most cases. However, they are
different things, and you may experience problems connecting
You will need a connection cable, which is an
optional part for some GPS models. Check your GPS documentation
for more details on activating the NMEA interface and selecting
the messages. Some GPS devices disable the dataport, to save
battery. You probably will have to change the default configuration
to enable the GPS data output.
To open the GPS interface dialog click the GPS Interface
button. A window will show, with current GPS position. Clicking
the Settings button will show the GPS settings
page, as shown below. Clicking again hides the settings.
- Set the baud rate to the same value as your GPS device.
Most GPSs have a default baud rate of 4800.
- Choose computer port number.
- Click the "connected" checkbox to open the
communications port and start receiving GPS data.
The upper panel will show the current position, date/time
and position status (as reported by GPS device).
The GPS interface can be set to save a position periodically
(AutoSave feature). Positions are saved to the current GPS track.
A given desktop can have only one GPS Track. Use the "Choose
GPS Track" button to choose the GPS track. If starting a
new travel, use the "New track" button.
You can also save positions manually, by clicking the "Save
position!" button. Just make sure the current position status
|Tip: If you use RMC
messages, Navigator will plot a small boat in the current
position, pointing to the actual course. If you use GLL,
which has no course information, a square will be plotted.
PC/GPS connections are sometimes tricky. A problem of either
PC or GPS settings will unable the communication.
First, make sure that the GPS device is transmitting data. Stop
the Navigator and open the Windows terminal (or Hyperterminal
in Windows 95/98). Set terminal communications settings to 4800
baud, 8 data bits, parity None, 1 stop bit. If the GPS device
is set correctly, you will see text messages coming from the GPS,
The first is a RMC message and the second a GLL. If you don't
see any message, the GPS is probably not sending data. You may
have to activate the GPS dataport and/or the transmission of each
message type (Some GPS devices disable the data port every time
they are turned off). After you start receiving one of these messages
(choose RMC if available), return to the Navigator and make sure
the correct data port, baud rate and NMEA message type are set.
You may set the Navigator's GPS interface to beep on each incoming
When planning a route between two points far away (over 1000
NM), many issues must be carefully considered: sea currents, prevailing
winds, ship routes, shallow and dangerous areas, foul weather,
etc. While these points are very important, there are also geometric
considerations: the Earth is a sphere and routes are not straight
lines but arcs in the spherical surface. Two kinds of routes are
of special interest: the lines of great circle
and the rhumb lines.
A Line of great circle (LGC) is the shortest
path between two Earth points. This kind of route is contained
in a plane defined by three points: the two route end points and
the center of the Earth. When plotted in a Mercator chart, a LCG
is represented by a curve, and a straight line in Gnomonic charts.
While LGCs are shortest, they have a few problems:
- True course changes constantly from point to point.
- Depending on the end points, a LGC can take you to high latitudes,
which is sometimes undesirable.
Rhumb lines (RL) are routes with constant true
course. They are represented by straight lines in Mercator charts
and curves in Gnomonic charts (the opposite of LGCs). RLs are
easy to navigate because the course is constant. They are, however,
a little longer (the difference is larger in higher latitudes).
Navigator Leg Calculator will calculate
both LGC and RL routes between the two end points. To open
the Leg Calculator, press the calculator button in the toolbar
or choose 'Routes, Leg Calculator' in the menu.
Besides distance and course, Navigator shows the difference
between the two options and builds routes with specified
number of segments.
These routes can be inserted in the current desktop. Routes
are added to the desktop with names "Rhumb Line"
and "Great Circle". You may change these names
in the route editor.
In the picture to the left, the yellow line is the RL route
and the red one is the LCG (both created with 22 points).
The difference between the two is 41.8 NM (1.26%). The LGC
route has a crosstrack difference of 241 NM to the RL route
(that is, it goes up to 4░ south of the RL).
When building the routes, LGC route points are set by the
program at constant longitude increments. RL route points
are set at constant distance increments.
The ChartMaker program was designed to work with chart
images in formats GIF, JPG and BMP, and prepare them for use in
the Navigator chart viewer.
This program has two main functions:
- Importing a raster chart images, producing CID files.
- Making vector charts (i.e. digitalizing points and polygons),
producing CHT files.
A computer image is formed by a large number colored dots - also
known as pixels - disposed in a rectangular mesh. If the image
is from a nautical chart or satellite photo, an algorithm can
be designed to find the real world coordinates of any of these
pixels and vice-versa. This is exactly how ChartMaker works.
The first step to make a vector chart or to import a raster chart
image is to establish the scale model between the image and the
real world. To do this, you have to click 3 reference points
and enter their world coordinates (Lat/Lon pair). This is enough
to define a scale between the real world and the image pixels.
Technically, when you click the three
reference points and enter the Earth coordinates, you
are actually defining four vectors: two in screen coordinates
(S12 and S13) and two in Earth coordinates (E12 and E13).
Since both geometries are similar, for any point P, we
SP = S1 + k * S12 + r * S13
EP = E1 + k * E12 + r * E13
SP = Screen coordinates of point P
EP = Earth coordinates of point P
S1 = Screen coordinates of reference point 1
E1 = Earth coordinates of reference point 1
r and k constants
From relation (1), we can calculate constants
k and r. With these constants, we can calculate
the earth coordinates of P from relation (2). This is
how the program calculates the earth coordinates of any
screen point (and screen coordinates of a Lat/Lon pair).
The method above works for linear latitude and longitude scale
charts. But most navigation charts (and particularly the detail
charts) are Mercator projections. Mercator projections have a
great advantage: distances and courses between points can be measured
directly from the chart. Angles are true, no correction is needed.
As any coast navigator knows, distances in these "regular"
charts must be measured in the latitude scale, one minute of latitude
being one nautical mile. To allow this, the Mercator projection
latitude scale is not linear.
If you look at the picture below, you will see that, as latitudes
get higher, distance between parallels in Mercator charts expands.
The size, measured in screen pixels, of a given latitude minute
is proportional to 1/cos(Lat). For instance, when Lat=0, cos(0)=1.0.
For Lat=60°, cos(60)=0.5, and so a minute of latitude at this
point twice as large as at Lat=0. Of course this "latitude
grow" is variable, and integral calculations are needed to
calculate the size factor between two latitude intervals.
Navigator's screen chart representation is not
a Mercator chart. The latitude scale is linear and the
factor between latitude and longitude axis tick sizes is
equal to Mercator projections in the center of the chart
window. This is, however, of no consequence, since you are
not going to "measure" anything on the screen.
You will use programs calculators and measurement tools,
which use the mathematically perfect Lines of Great Circle
(LGC) for distance and course calculations.
Still, Navigator knows how to deal with Mercator
chart images. When using a Mercator chart image on the Navigator,
remember to check the Mercator latitude scale checkbox.
The program will correct for the Mercator scale automatically.
- Mercator chart images are OK, even if the image is rotated
(like chart 1 on the left). It's difficult to perfectly
align a large chart when scanning. ChartMaker will
rotate the chart in order to align its grid with the vertical
direction (chart 2). This is necessary to speed up chart
resampling, for displaying.
- Gnomonic and other curved grid projections cannot
- Some images (particularly satellite images) are skewed.
Their constant latitude and constant longitude lines are
not orthogonal (like chart 3 on the left). These images
cannot be used. After entering the three reference
points, Chartmaker will inform you if this kind of problem
- Scan using resolutions between 75 and 150 dpi. Avoid large
files (over 1MB GIFs or JPGs). Images will have to be ressampled
by the Navigator at run time while zooming. Large images will
consume a lot of CPU cycles, resulting in poor and bumpy performance.
- When scanning large paper charts, you may detach the page
scanner cover (if the scanner allows) to avoid folding the chart.
Put some weight over the scanner cover, so that the paper is
flat and perfectly in contact with scanner bed.
- If the chart doesn't fit in the scanner bed, scan it in parts,
overlapping between them. Then make a CID file for each image.
Navigator can display different charts together, and will even
allow partially opaque rendering, merging the overlaps.
- Larger scanners (A3) are preferred, when available. A4 scanners
are small for large paper charts.
- Hand scanners are not recommended, because they can distort
the image scale. Use only page scanners.
- Images should be saved in GIF or JPG format (JPG is usually
The 3 reference points indicate how the pixels in the chart image
map to the actual world. Care must be taken in choosing these
points, in order to achieve a precise scale. Below are some guidelines
for choosing the reference points:
bad points- too close!
Choose reference points that are as far from each other
as possible. This will result in a more precise scale.
If possible, choose points near the corners of the image.
In the image on the left, points are too close and the scale
will be poor.
bad points- nearly aligned!
Reference points must not be aligned. In the image
on the left, points are nearly aligned and the scale will
good reference points
In the image on the left, points are well positioned (i.e.
far from each other and not aligned). Choose points that
are close to the corners of the image, if possible.
If the chart has a grid, the grid line crossings are logical
candidates for reference points (like point 2). But other
points can be used and they don't need to define a right
angle between them.
- Open the chartmaker program and click Import chart
load the image to be imported.
- Check the Mercator scale checkbox, if chart is a Mercator
projection (most are).
- Take a good look in chart and choose the 3 reference points,
as described above. Click
each reference point with great care. Inform the precise latitude
and longitude for the point.
- Chartmaker will then calculate the scale and produce a summary,
rotating the image as needed (rotation is considered necessary
if the angle between a meridian and the vertical is larger than
- If the image was rotated by ChartMaker, you must save it.
I use to save the image with a different name, keeping the original
scan intact, but you may save with the same name, ovewriting
- If you made any mistake or if the image is unsuitable for
use, the summary will inform you of this. You may try again,
if you think the image is suitable and you made a mistake.
- After the summary, you can click extra points in the chart.
This is optional, but highly desirable, to further check if
any error was done in the reference points.
- If everything is OK, save the CID file (Chart Image
Definition). This text file contains the information for the
Navigator program to use the image: the reference points data,
image file name and description.
- The CID file also contains the image digital signature (of
type MD5). This is to make sure that the associated image is
unchanged. So, if you do any changes in the image - even changing
a single pixel color, the CID file will become invalid. This
is a security feature to avoid accidental image change.
While raster charts are very complete, with the all relevant
navigation information, vector charts cosume less memory, are
a lot faster to draw and are "zoomable". This is why
is interesting to have a vector chart to be loaded underneath
the local raster charts.
Navigator uses a custom chart file format, with the CHT
extension. These are plain text ASCII files and can be edited
with the Windows Notepad or Wordpad. Because providing detailed
vector charts of all regions of the Earth is a demanding task,
I choose to give users a tool to roll their own charts. Producing
a vector chart means using a raster chart image to digitalize
the points in the shoreline.
Chartmaker accepts three image formats: Windows BMP, GIF and
JPG. If you are scanning paper charts, JPG is probably the best
format, because files are smaller. It's a compressed format.
The CHT file
Once you have the image of the chart, load it in Chartmaker.
Now you have to define the scale for this chart. Click and enter
the Earth Coordinates for three distinct reference points
of the image, as described above.
- Choose three points that are relatively distant to each other
(at least 1/3 of the screen). Select points that form a right
angle. Click the points and enter their Earth coordinates.
- After the third point, the caption will show the coordinates
of the cursor, as you move the mouse. Check the coordinates
of other points, to certify that your scale is accurate.
- Select "Edit, Draw Axis" in the menu. The program
will draw vertical and horizontal axis in 30' intervals. Check
if they are the same as the image's axis. Draw again to erase
- Now draw the shoreline, clicking the points with the mouse.
Draw different polygons to represent the islands and continents.
- After clicking all points of a given polygon, press the "close
polygon" button. The last point will be connected to the
first. All polygons must be closed.
- Give a name for the polygon.
- After drawing all the polygons, save the CHT file.
- Open it in Navigator chart viewer, to see the result. Zoom
in and out, to check the details.
In the figure to the left, the 3 reference points are marked
1, 2 and 3. The coordinates are:
1) 24░00.0S 46░25.0W
2) 23░50.0S 46░25.0W
3) 24░00.0S 46░06.6W
If you open the supplied myworld.cht file in Navigator,
you will see a simplified chart of the whole world. I made
it more detailed in one part (44░00'W 23░30'S), the region
I usually navigate. You can do this for your region, by directly
editing the CHT file with a text editor and adding the polygons
of your region. This is a little tricky. You might prefer
to roll a chart of your region only.
You may also merge CHT files, using an ASCII text editor (like
the Notepad or Wordpad). Take a look in the CHT file header for
more technical details on the CHT file format.
(c) Copr. 1991-2001 Omar F. Reis - All rights