Preparing the Raster Map
Before starting the georeferencing process, it is important to ensure that the raster map is of good quality. The image should be sharp, free from scanning distortions, and contain cartographic features that can be used as control points.
If the map has been obtained by scanning, it is advisable to use an adequate resolution and check that there are no folds, distortions or cropped areas that could compromise calibration accuracy.
Maps that include a kilometer grid, geographic coordinates along the borders, or other clearly identifiable cartographic references are particularly suitable.
The choice of control points directly affects the accuracy of the georeferencing process. When several alternatives are available, it is recommended to follow this order of preference:
- intersections of the projected coordinate grid (UTM, Gauss-Boaga, etc.);
- intersections of the geographic graticule (latitude and longitude);
- well-defined and easily identifiable features on the ground, such as geodetic survey markers, road intersections or bridges, only when the map does not include a coordinate grid.
When a map includes a coordinate grid, grid intersections are almost always the most accurate control points. Avoid using cartographic symbols such as mountain huts, springs, churches or other features represented by icons, because, for readability reasons, they may be slightly displaced from their actual position.
In OkMap, the accuracy of control point placement can be improved by using an appropriate zoom level or the magnifying tool. This is particularly useful when selecting grid intersections.
Selecting Ground Control Points (GCPs)
Ground Control Points (GCPs) are locations on the map whose geographic coordinates are known with precision. They provide the link between the raster image and the real-world reference system, allowing the software to compute the transformation required to georeference the map.
When the map includes a projected or geographic grid, grid intersections usually provide the best Ground Control Points, because their coordinates are printed directly on the map. Only when no grid is available should you use clearly identifiable features such as geodetic survey markers, bridges or road intersections.
It is preferable to avoid features whose mapped position may only be approximate, such as mountain huts, springs, isolated buildings or other conventional map symbols. Although they are easy to recognize, they do not always coincide with their actual location and may reduce the accuracy of the georeferencing process.
For accurate georeferencing, it is important to distribute the GCPs across the entire map rather than concentrating them in a single area. A uniform distribution allows the software to compensate more effectively for any distortions in the original map.
Assigning the Correct Coordinates
After identifying the Ground Control Points, you must assign the correct coordinates to each one using the map's reference system. The coordinates may be expressed, for example, as latitude and longitude or in the UTM coordinate system, provided they are consistent with the map being used.
Choosing the wrong datum, map projection or coordinate system can shift the entire map away from its real position. For this reason, you should always verify the information printed on the map before starting the calibration process.
Once the control points and their corresponding coordinates have been entered, the software automatically computes the geometric transformation required to align the image with the selected reference system.
Checking Calibration Accuracy
Once georeferencing has been completed, it is advisable to verify that the map is correctly aligned by comparing it with reliable geographic data such as GPX tracks, waypoints or already georeferenced maps.
If the entire map appears to be consistently shifted, the cause is often an incorrect datum, map projection or coordinate system. Local errors, on the other hand, are usually caused by poorly chosen Ground Control Points or distortions in the original map.
Before considering the calibration complete, it is good practice to check several points distributed across different areas of the map to ensure that alignment is consistent throughout the georeferenced area.
Using Georeferenced Raster Maps
Calibrated maps can be used to display GPX tracks, waypoints and outdoor routes with geographic accuracy.
This makes it possible to integrate historical maps, scanned hiking maps and custom images into GPS software.
Georeferencing Maps with OkMap
OkMap provides all the tools needed to georeference raster maps using Ground Control Points (GCPs), supporting a wide range of coordinate systems, geodetic datums and map projections.
During calibration, the software guides the user through entering the control points, automatically computes the geometric transformation and allows the result to be verified by overlaying the map with GPX tracks, waypoints and other geographic data.
Once georeferencing has been completed, the map can be used for navigation, GPS track analysis, route planning and the creation of custom maps, while maintaining proper alignment with other geographic data.