How to Calibrate Your GPS and Improve Map Accuracy

A GPS device is a radio receiver and specifically NOT a transmitter. It receives signals from GPS satellites, processes the data, and reports your location in terms of latitude, longitude, and time. Calibration, however, is more about understanding the limitations and improving the accuracy of the map you are using rather than adjusting the GPS device itself.

Calibrating Maps with Geodesic Points

The calibration of a map involves checking the geodesic points. These points are established by your national agency and serve as a reference for precise coordinates. For global scales, international points like those used by the International Earth Rotation and Reference Systems (IERS) and the International Terrestrial Reference Frame (ITRF) are utilized. These points are designed to be easily identifiable on satellite and aerial images.

Understanding the Scale of Your Map

The scale of the map is defined as the ratio at which 90% of geodesic points are 0.9 mm or less from their actual location when measured on the map. However, it's important to note that the typical errors in Google Maps for geodesic points are around 10 to 15 meters, especially when using aerial images, and can be as high as 20 to 100 meters in satellite images.

Working with Satellite Images

When working with satellite images similar to Google Maps, you may encounter issues with orthographic correction. The images taken by airplanes are not perfectly vertical, and the plane is not directly over the objects being photographed. This means that objects to the sides of the airplane appear distorted. Therefore, the scale of the map can vary significantly depending on the quality of the images and their orthographic correction.

For example, if you are using high-quality aerial images with 2 to 5 meter pixel resolution, the approximate scale of Google Maps could be around 1:10,000. Conversely, if you are working with satellite images with 50 meter pixel resolution, the scale could be around 1:111,000. These scales are estimates and can vary depending on the specific conditions of the image acquisition and processing.

Understanding Human Error in Cartography