I bought a set of 1:50000 Gabon map issued by french IGN (Institut National de Geographie), they are image in JP2 format, they do give information about ellipsoid, projection, graticule and grid, but I am unable to precisely registering them, I always have about 200m shift if I compare with google earth image. This range of precision matters since I need these maps to locate hydroelectric potential sites.
The legend says : Universal Transverse Mercator zone 32N, ellipsoid of Clark 1880
Here is what I do :
1. create a new drawing, assign projection of these drawing in UTM 32N, Clark 1880 IGN (cf image attached)
2. put on this drawing as many points as the graticule let me do so (e.g 16 point with 5min space)
3. export the image in png, import it back to decompress the data (in order to get more registration methods)
4. put the corresponding control points both on the drawing and on the image
5. register (I tried almost everything : affine (scale shift, rotate), with and without scale pixel equally in X and Y direction, triangulation, and affine (scale shift))
When I control the result, the status bar gives me both correct lat/long and projected coordinate (cf file status bar.jpg)
When I create a map with both my control drawing and my image things seems ok, I have almost no shift (shift is below 10m which is smaller than the size of my pixels, cf image attached)
When I control with google earth, things goes wrong, the last image shows in purple a river roughtly traced on my control drawing from google earth, there is a shift of 250m :(
What is wrong?
The set of map is quite old (1960-1980) but I have a colleague who succeed doing the same thing with maps of Benin from IGN... Is it Google earth that is not accurate, are the IGN map not accurate? Something went wrong in my registering process?
There are other ellipsoid Clark 1880 in the projection database, I tried one but it gives the same result
I have 50 of these maps, it costed me about 350EUR, it is very deceitfull that I cannot register them with manifold...
PS I haven't tried the numeric method for registration, takes too long with my old computer (>1hr)
map control drawing - registered image.JPG
UTM 32N Clark 1880.JPG