There are several ways to capture GCPs. Let us run through them -
GCPs captured in the site -
This is the most common and reliable way to capture GCPs
This requires the use of high accuracy instruments placed on pre located points at the site.
The main requirement for this type of GCP capture is accessibility.
GCPs from existing sources - It is quite possible that there are existing georeferenced maps that your customer trusts
and uses for the project at hand
If they do, it�s best to request access to them and use certain known check points at site as GCPs.
After all, your outputs need to be overlayed with their existing trusted maps,
so what better source for GCPs than using their maps!
GCPs from Web Map Services -
A Web Map Service (WMS)
is a standard protocol for serving (over the Internet)
georeferenced map images which a map server generates using data from a GIS database. Many web map
services have their data freely available to public.
Some popular web map services are Google Maps
and Bing Maps ..The accuracy may vary in different parts of the world
so be sure to double check before using this is the source for GCPs.
This is most useful when your customer is primarily going to use the kml output to align with Google Maps.
We recommend using daft logic sand box as the source for obtaining 3D GCPs
for your projects, if you are going to use WMS as the source.
On clicking on the point of interest, the 3D coordinates appear in the box below the map.
The coordinate system that is used for capturing the GCPs has to be the same as the desired coordinate
system of the project.