Calibrating a Scanned Map

Before a scanned map can be displayed in the correct location, it needs to be calibrated (geo-referenced). This process involves specifying the location of two or three known points on the map. Once the correct geographic coordinates are known for these points (called Calibration Points), ExpertGPS can calculate the coordinates of every other point on the map.

Your map must meet the following requirements:

  • The map must be in JPEG, TIFF, PNG, or BMP format.
  • The map must be oriented so that North is up. If the North arrow is more than 45 degrees from vertical, you will need to rotate the map before proceeding.
  • You need to know the projection used to create the map. This information is usually printed at the bottom or in the margin of the map.

Tip

  • For maps that only cover a small area (your town, e.g), you may get reasonable results without knowing the projection used when the map was created. Use the default Equirectangular projection. This method relies on the fact that the Earth is pretty much flat when you're only considering an area of a few square miles.

The calibration process will be easier if you create a GPX file containing waypoints and a track or two from the area shown on the map. When your uncalibrated map is first displayed, ExpertGPS will position it so that the current GPX file fits nicely on top of the map. If the current file contains data from the area shown on the map, you will be able to verify that you have calibrated the map correctly, because your data will line up with the corresponding features on the map.

To view the uncalibrated map, do the following:

  1. On the Map menu, click Show Scanned Map.
  2. The Select Scanned Map dialog will appear. Select the map you just added to the Scanned Map Library.

The uncalibrated map will appear. Because the map is not yet calibrated, it will appear in the wrong location, relative to the GPS data in the file. You will now add calibration points to the map, which will "anchor" a specific landmark on the map to its correct geographic coordinates.

To begin calibrating the map, do one of the following:

There are two ways to enter a calibration point. If your map has a grid overlay, you can click the Calibration Point tool on the grid intersection, and enter the coordinates of that grid intersection. If you already have waypoints for known features on the map (trailheads, summits, a road intersection) you can hold the Shift key and click on the map feature. You will be presented with a list of waypoints - choose the one that marks the feature you are using as the calibration point.

Two calibration points are generally sufficient to geo-reference a topo map or nautical chart. If the lines of longitude on your map are not perfectly vertical, or if your map is rotated, you may need a third calibration point to correct for the rotation.

Setting the first calibration point.

Setting the second calibration point.

Setting the third calibration point.

When you are finished calibrating a scanned map, do one of the following:

  • On the Map menu, click Check markCalibrate Scanned Map.
  • Close the Scanned Map window.

The Keep Scanned Map Calibration Changes dialog appears. If you are not satisfied with the calibration point changes you just made, you have the option to discard them.

Keep Scanned Map Calibration Changes Dialog

Your scanned map is now calibrated and ready for use. The next time you want to view GPS data over this map, click Show Scanned Map on the Map menu.

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