Tracing raster maps
It is often very valuable to use geo data from old paper maps in new interactive map. Unfortunately in most cases to achieve this goal user has to install complicated GIS tools, calibrate maps etc.
ShareMap new raster overlay feature address these problems with ShareMap you can upload any scanned map in
Uploading and calibrating
Before calibration, you have to had base file in PNG or JPG format. File dimesions should not exceed 5000px x 5000px. If you want to to trace only part of map please crop it before uploding (map with smaller dimensions work faster).
The best performance is achieved with maps 1500px x 1500px and smaller.
After uploading map add calibration points - the points that you place both on scanned map and interactive map.
Minimum count of calibration points (GCP) id 3, but more is better. After adding calibration point click Apply changes. At this moment user have to wait a little bit, because map is uploaded to ShareMap server and processed by GDAL library at server side.
Using calibrated map
After map is calibrated it appears on user interactive map. User can draw freely on mapy using lines and points from raster map as guides, after everything is finished user can remove raster map using Remove raster map button.
Remember about map copyrights, many old maps are already free, other espacielly created by United States goverment are in public domain space at moment of creation. Some minor tracing can be done under fair use law. Always consider copyrights before tracing map.
You can video tutorial of calibration process here.