Hello<br><br>I had a similar problem with some old scanned maps from Portugal that were in a local topographic coordinate system. I also wanted to have a PROJ.4 string for that system in order to load the maps in a GIS software together with other data sets. I did it simulating a LCC projection (Lambert Conformal Conic).<br>
<br>First there is an assumption we can do about the affine transformation. It is likely that it is nearly a conformal transformation (a=e and d=-b) and with a scale factor very close to 1 (sqrt(a*a+b*b)=1). In this case the essential thing to deal with is the rotation of the axes.<br>
<br>In the LCC projection you can choose one standard parallel in your region so that the linear deformation is always very close to 1. Then you can choose a central meridian far enough, so that the cartographic north (the Y axis of the projection) becomes parallel to the Y axis of your local coordinate system. You can do that by trial and error or by analysing the meridian convergence of the LCC projection. Then you can add a false easting/northing so that the projection and your local system coincide.<br>
<br>Suppose for example that point (4.8E, 52.2N) is the origin of your local coordinate system and point (4.9E, 52.4N) is on the Y axis.<br>Consider 52.3N as the standard parallel. The following PROJ string converts these two points in a way that they have the same easting:<br>
<br> +ellps=bessel +proj=lcc +lon_0=-16.68875 +lat_0=52.3 +lat_1=52.3 +lat_2=52.3 +no_defs<br><br>The projected coordinates of the first point are (1447575.65 m, 205246.75 m). If you add symmetric values as false easting/northing<br>
<br> +x_0=-1447575.65 +y_0=-205246.75<br><br>the PROJ string does what we want. You can also play with k_0 if some scale adjustment is still needed.<br><br>This simulation of a projection is obviously only valid for rather small areas. It worked well for me, in Quantum GIS, with the "on the fly" option.<br>
<br><br>Regards<br><br>Jose Gonçalves<br><br><br><br><div class="gmail_quote">2013/1/2 Jan Hartmann <span dir="ltr"><<a href="mailto:j.l.h.hartmann@uva.nl" target="_blank">j.l.h.hartmann@uva.nl</a>></span><br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<font face="Times New Roman, Times, serif">Hi Frank and all, happy
new year!<br>
<br>
Is the following possible with Proj? I am working with historical
maps that are rotated with respect to a well known
epsg-projection, see for example<br>
</font><br>
<a href="http://mapserver.sara.nl/a1000/bladgrenzen.html" target="_blank">http://mapserver.sara.nl/a1000/bladgrenzen.html</a><br>
<br>
<font face="Times New Roman, Times, serif">The blue squares are the
sheets of the map 1:1000 of Amsterdam after 1940, in epsg:28992,
the red squares are the same sheets before 1940 in a local
coordinate system. I know the conversion coefficients for the two
coordinate systems: a simple six-value transformation of the form:<br>
<br>
xnew = a*xold + b*yold + c<br>
ynew = d*xold + e*yold + f<br>
<br>
I would like to access the second map via a proj-string. For that,
I would have to expand proj with a procedure that takes the six
coefficients (a-f) and generates the transformed coordinates after
the regular transformation.<br>
<br>
Is this possible? Can anyone give me some pointers where to look
in the proj-code? Conceptually, it looks simple to me, but I don't
know where to start.<span class="HOEnZb"><font color="#888888"><br>
<br>
Jan<br>
</font></span></font>
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