Car GPS Navigators
Topo Map Calibration with a the help of a car GPS? Car GPS Navigators know a lot about all roads on their maps and they certainly know the exact GPS coordinates of cross roads. If they allow you to read the GPS coordinates of any point on their maps, you will have a precious source of information for the area of your topographic map. For any, except the most remote, area you can find several cross roads, known by your GPS Navigator.
With TomTom Navigator 5 and Destinator 6 on the pocketPC we could not figure out how to obtain this kind of information, but with our Navman iCN 510 it was really simple. When you hold the stylus on the map screen, it indicates the coordinates of the point of your choice.
Topographic map to be calibrated
The map below is a wonderful map of a small part of France, but the longitudes and latitudes are indicated in Grads, instead of Degrees and the origin in not in Greenwich, but in Paris. We want to calibrate this map to latitudes and longitudes in WGS84 degrees.
On the map we have already indicated five calibration points. We will show you where they came from.
Map extract from Carte Topographique TOP 100 51 Lyon Grenoble 1 : 100 000.
© Institut Geographique National www.ign.fr 2006.
The chosen calibration points
Not that we need five points for Topo Map Calibration of this map in OziExplorer, but with more points, we sort of average eventual small errors in each point (or a bigger error in one of the points). The screenshots are from the Navman iCN 510. The images on the left are an overview and the images on the right the detail presentations.
Point 1
Point 2
Point 3
Point 4
Point 5
The coordinates as read from the screens
The iCN 510 gives the seconds with only one decimal, contrary to Google Earth two decimals. For OziExplorer we need the coordinates as Degrees and fractional Minutes, so we arranged that in the table too.
The calibrated Topographic map
Here is the calibrated topographic map, "ready for GPS".
Map extract from Carte Topographique TOP 100 51 Lyon Grenoble 1 : 100 000.
© Institut Geographique National www.ign.fr 2006.
Test of Topo Map Calibration
Now we want to test the result of our calibration effort. For this we take a detail of our calibrated map. West of Crémieu we see a roundabout on the crossing of the D75 et D24 (See red arrow). On the map the center of the roundabout is indicated with a dot. In OziExplorer we measure the GPS coordinates of this dot on our calibrated map. We measure the exact same point in a High Resolution Google Earth image for comparison. The results are in the table below.
Map extract from Carte Topographique TOP 100 51 Lyon Grenoble 1 : 100 000.
© Institut Geographique National www.ign.fr 2006.
Comparison with Google Earth
In the table below we see the GPS coordinates of the same point on our calibrated topographic map and in Google Earth.
Conclusion
In the table we called the calculated distance between these two measurements "Error". Maybe this is not the right word for this difference. This could very well mean that Google Earth is 8 meters more "off" than our topographic map, calibrated with the Navman iCN 510.
Anyway, it seems that this method is another alternative for a precise Topo Map Calibration.
