Hello,
Project ShareMap[1] uses gdal_warp on the server side to allow users
to work with calibrated rasters maps as overlay on web mercator maps.
In case of maps in large scale it performas relatively well (country
maps - almost ok, provinces maps - perfect)[2]. The problem is if we
try to calibrate small scale maps (like this historical europe map)
using only GCP.
This map[3] is a good example - if we split it to separate countries
and calibrate every country alone, result is ok, but I don't know how
to calibrate entire map with only GCP's (map does not contains
informations about projection)
ShareMap currently execute such commands to warp map to web mercator:
gdal_translate -of GTiff -gcp 281.0 1290.0 -606604.2360752566
4290256.933470774 -gcp 832.0 1512.0 1213208.4721505132
3982062.8778166603 -gcp 1722.0 1668.0 3896453.822854374
3441500.2881376203 -gcp 1436.0 1021.0 3226253.9813841325
5618426.55426589 -gcp 653.0 701.0 168772.95277900095 6670199.918799774
map.jpg map.referenced.tif
Values of GCP are in in meters for WebMercator - but in fact this map
does not look like mercator projection.
Next step is warping
gdalwarp -t_srs +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0
+lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs
map.referenced.tif map.referenced_warped.tif
My question is very short - what is the best way of georeferencing and
transforming to web mercator map with unknown projection, where we have
several (4 or more) pairs of LAT/LNG and X/Y of pixel.
LAT;LNG;X;Y
35.92464;-5.44921;281;1290
33.65120;10.89843;832;1512
29.51610;35.00244;1722;1668
44.98033;28.98193;1436;1021
[1] ShareMap - ShareMap.org is new web mapping platform that allows
author to create maps on free sources using data from OpenStreetMaps
and trace from calibrated raster maps
[2] See this movie for example - www.youtu.be/gPJ7AD15wRI
[3]
http://www.romanscotland.org.uk/pages/infrastructure/maps/RE395adL.jpg
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