Calibrate Map & Display GPS Information

By Robert J. Ballantyne

This article describes how to combine the data from a GPS receiver with computer and maps. Specifically, how to:

  1. Display field-recorded tracklogs on a map
  2. Display field-recorded waypoints on map
  3. Upload waypoints to the GPS receiver
  4. Upload routes to the GPS receiver

The key to each of the four processes is a properly calibrated map in your computer.

The process uses the following equipment: The eTrex Summit GPS receiver, Macintosh computer & scanner, Garmin-to-USB connection cable.

And the following software: MacGPS Pro, GraphicConverter, any scanning software, Canadian 1:50,000 topographical maps produced by the Canada Centre for Mapping.

Primarily, this is to demonstrate the process. There is software that can work for people with wintel computers, and the principles would be similar with other receivers.

Calibrating a Map

After you have a calibrated map, then you can are ready to upload, downlown and display GPS data. First, here is how to calibrate a map.

Scanning

Unless you have the map available on a CD, you will begin with scanning the paper map. Large images take lots of disk space and computer memory. Usually you will scan only the parts of the maps that interest you.

Since maps look at large areas, and the GPS can have an accuracy of less then 9 meters, sometimes you will be looking at such tiny parts of the map that the features are just a few pixels. The web uses 72dpi, and I find that a resolution of twice that is adequate for maps. At 144dpi a pixel is represents about 8 meters. All this means is that care needs to be taken to make the scan as flat as possible. My used maps have been folded and refolded several times. It is also awkward to stuff a big topo map in a small scanner. To keep it really flat I usually pile a copy of the Compact Oxford Dictionary on the scanner lid.

Regardless of the scanner's preferred format, the image must be saved as a PICT file.

Rotate Image

The map calibration software does not have any image manipulation ability, so software like Photoshop is essential. Almost all such programs will let you tweak the colour, brightness and contrast, if necessary. For GPS, it is critical that the north-south lines be precisely vertical. It means that after scanning, the image usually needs to be rotated. The program, GraphicConverter, is tiny compared to Photoshop, and has a useful technique for rotating images. It allows you to drag a line on the image, and then the image is rotated so that the line will be vertical or horizontal (whichever is closer). The blue UTM grid is a feature of Canadian Topo maps. The rotation-line can be carefully dragged along one of the UTM lines. Taking the time to do this carefully will improve the accuracy of the GPS plots. After rotating the image I trim it so the sides are parallel to the grid.

I am sure that the CD editions of the maps would be better for flatness and rotation. Stitching two or more maps together is a tedious job, and I understand that the CD maps avoid that problem too.

Use the UTM Grid

In school you were taught how the global coordinate system of Latitude (lat.) and Longitude (long.) works. Your GPS unit will happily receive and report positioning in lat. and long. should you wish. With the Canadian topo maps, you will find that the lat. & long. information is printed around the perimeter of the map, and it is usually very difficult to ascertain a position in the middle of the map. It is impossible if you cannot spread it on a chart table, or if the map is folded. In other words, finding a position by using lat. & Long. is useless in the field. Fortunately, these maps all have the Universal Transverse Mercator (UTM) grid printed with blue lines across the entire map. Each square in the grid is exactly one kilometer on a side. This is such a useful feature of topo maps, that once you have discovered it you will use it to describe places and estimate distances, even if you do not care about GPS.

Each map in the UTM grid shows part of a ZONE. The software will need to know the Zone of the map to be calibrated. The Zone is printed in the middle of the right margin. All of the maps near Vancouver are Zone 10 U.

When you have a PICT image of the map, and it is rotated so that North is up, and you know the Zone of the map, you are ready to calibrate the map.

Run MacGPS Pro. (The author upgrades regularly at no charge to users. At the time of writing I am using Version 4.0.3.) Since you have no map loaded, all that you will see is the menu bar. For this example I am using 1976 edition of map 92 J/11 called, North Creek. The last picture below is an calibrated segment of that map.


Find Map Information

To the right of the scale at the bottom of the map I see that the contour interval is 100 feet. In that same section of small type I learn that the map is North American Datum (NAD) 1927, Transverse Mercator Projection. In a block in the right margin of the map it explains: Grid Zone Designation 10U.

Select OPEN...

And Open the rotated PICT image of the map to be calibrated.

Before the map appears two dialog boxes will need information about the map.

At the first dialog box, fill out the following information about the topo map:

  • Geodetic Datum. Most Canadian topo maps are NAD27 Canada, or NAD83 Canada. This map is the former, so I select that.
  • Select UTM/UPS Grid
  • Select Transverse Mercator
  • Select OK.

A second Dialog box appears with lots of empty fields. If you fill in the first box asking for UTM Zone: it will fill in everything else that is needed here.
  • In this case the zone is 10 U
  • Select OK

Calibration Map

Two windows open. One has the MAP and the other has the calibration FORM. Expand the map window so that, without hiding the form, it is large on the screen. To calibrate, you will need to identify a triangle of 3 widely spaced known points. Use the intersections of the UTM Grid. The buttons on the right of the map allow you to zoom in and out. Dragging a rectangle will allow you to zoom in quickly to a spot.

Using the map, identify the 3 points, and zoom in on the first. To plot the first point, carefully click on its position in the centre of the intersection of the blue lines. Try to be accurate as possible. Zoom in until you can see the pixels of the line. Here the blue UTM lines are 2 pixels wide. The software will mark the spot where you click. If you don't like the place where you clicked, simply click again in a better spot.


Map Calibration Form

Enter the UTM coordinates of the position you marked in the calibration form. The horizontal blue lines are called Northing. In this example, if you follow the line that is marked to the edge of the map it is marked 06. By looking at the corners of the map you will discover that 06 are the significant digits in a larger number, and the Northing is actually 5606000. Enter the complete number. The north-south lines are called Easting. The significant digits for this example are 99, and at the edge of the map you will see that the whole number is 499000. Enter that in the form. When you fill in the zone, 10U the OK button becomes available. Click OK. The form reappears blank. Repeat the process for the other two points. When you have marked 3 points the DONE button becomes available. When you click it, the map is calibrated.

Choose SAVE CALIBRATED MAP AS... and pick a suitable name. This is the map that you will use for all of the plotting. To see how accurate your plot is, select MAP > SHOW UTM GRID LINES.

The software's grid should perfectly align with the middle of the blue lines.

Be sure to save the calibrated map without showing any gridlines, or the lines will be saved with the image of the map. Later, when you wish to plot data on this map you can choose which grid to use (e.g. lat/long or UTM), or to ignore the grids.

Next...

How to Download, Upload and Display GPS Receiver Data