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GPS Mapping Software - Calibrating Maps - Tutorial

Introduction

In order to use ordinary image files as maps, the software has to map the image pixel coordinates to geographic coordinates. Before this can be calculated, you have to calibrate the map. Calibrating the map can be done in two ways:

• Calibrate by selecting some known coordinates on the map;
• Calibrate by setting the map's boundaries;

Calibrate by selecting two or more coordinates on the map

When calibrating the map by selecting known coordinates, you have to select 2 or more points on the map, and enter the coordinates for this points. The more points you select, the better the precision. Make sure you select the points at random places on the map. It does not make sense to select all coordinates with the same X or Y coordinate.

Step 1: Loading a map

First you have to load a map. The support image file formats are GIF, BMP, PNG, TIF and JPG. When you selected a map, the map will be displayed and a messagebox will popup, telling you that you should calibrate the map in order to use it.

Step 2: Selecting the reference points

Start by selecting the points that will be used for calibration. These can be known coordinates on a map ( for instance waypoints or landmarks ), or you can use the intersections of the grid that is drawn over the map ( if any ). To select a point, right-click on the map, and select the "Calibrate Point" option from the popup menu. The "Calibrate Map Point" dialog will appear. In this dialog you have to enter the Latitude / Longitude coordinate of the point you just clicked, and click the "Save" button. You have to select 2 or more points this way. These points should be evenly divided over the map.

Step 3: Selecting the map projection and finish the calibration process

After you selected the points, open the "Map Calibration" dialog. You can open this dialog, by selecting "File" => "Map Calibration..." from the menu. In the first section of this window, you can find the calibration points you just entered. By selecting them, and clicking on the right mouse button, you can modify or delete these points. Click the "Select" button to select a projection for this map. A projection is required. Most maps use Mercator 1SP projection on the WGS84 Ellipsoid. To finalize the calibration, click the "Calculate" button. If everything went okay, you can close the window and start using the map.

Calibrate by setting the map's boundaries

When you exported a map from for instance Google Earth or OpenStreetMaps.org, you probably know the area that is covered by the map. By just entering the the coordinates of the upper-left and lower-right corner of the map, you can simplify the calibration process. Please not that this method cannot be used for all projections. If you have a map where the grid lines are total horizontal and vertical (for instance: Mercator Projection), it can be used.

Step 1: Loading a map

First you have to load a map. The support image file formats are GIF, BMP, PNG, TIF and JPG. When you selected a map, the map will be displayed and a messagebox will popup, telling you that you should calibrate the map in order to use it.

Step 2: Selecting the map boundaries

When the boundaries are know, we do not have to setup any calibration points, these points will be created automatically. You have to open the "Map Calibration" dialog. You can open this dialog, by selecting "File" => "Map Calibration..." from the menu. Locate and click the "Bounds" button, a dialog box will appear, asking for the upper-left and lower-right coordinates of the map. Click "OK" to save the information, and you will notice that four calibration points ( using the four corners of the map ) are created.

Step 3: Selecting the map projection and finish the calibration process

Click the "Select" button to select a projection for this map. A projection is required. Most maps use Mercator 1SP projection on the WGS84 Ellipsoid. To finalize the calibration, click the "Calculate" button. If everything went okay, you can close the window and start using the map.