How to obtain the georeference using 2D or 3D GCPs taken from a Web Map Service

Although Ground Control Points (GCPs) are not necessary for processing, they improve the absolute accuracy of a project. Web Map Service (WMS) servers are a good alternative when GCPs are not available. These map servers provide online georeferenced maps using a standard protocol called Web Map Service. Some servers provide their GIS database free of charge. Two well-known WMS servers are Google Maps and Bing Maps. They cover the globe but the accuracy of the georeference might be low. Moreover, the accuracy of the data varies for different locations.

Warning: The accuracy of some WMS servers might be low. Therefore, it is NOT recommended to use them for projects that require high accuracy. Do not forget to adjust the accuracy of the GCPs in the GCP/Manual Tie Point Manager. For more information about the accuracy of the GCPs: 202557919.

To obtain the georeference using 2D or 3D GCPs from a WMS server follow these steps:  

 Getting 2D GCPs from Google Maps:

1. Open Google Maps in a browser:
2. Search for the location where the project lies.
3. Zoom in up to the closest view possible of the area of the GCP.
4. Move the mouse over the point where the GCP has to be marked. Open the context menu using the right mouse button. Click What's here? A box is displayed with the location and the coordinates.


Getting 3D GCPs from Google Maps Sandbox on Daft Logic:

1. Open Google Maps Sandbox on Daft Logic:
2. Search for the location of the project.
3. Zoom in up to the closest view possible of the area of the GCP.
4. Move the mouse over the point where the GCP has to be marked and click on the map. The 3D coordinates will appear in the box below the map.


Connecting to WMS servers inside existing GIS software:

As WMS is a standard protocol, it is not necessary for servers to create their own user interface to provide the map services. GIS software such as ArcGIS, Global Mapper and Quantum GIS allow the connection to WMS servers and browse the requested maps inside their application. This is another option to collect the required GCP coordinates using another source than Google Maps.


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