<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<font face="Times New Roman, Times, serif">Thanks Jose, I'll give it
a try<br>
<br>
Jan<br>
<br>
</font>
<div class="moz-cite-prefix">On 01/03/2013 02:47 AM, Jose Gonçalves
wrote:<br>
</div>
<blockquote
cite="mid:CALHLaORCfcumBJBvnc52Y_3ztD-ASjqC282+pNHf_fGX9c4a5w@mail.gmail.com"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=ISO-8859-1">
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
moz-do-not-send="true" 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">
<div text="#000000" bgcolor="#FFFFFF"> <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 moz-do-not-send="true"
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> </div>
<br>
_______________________________________________<br>
Proj mailing list<br>
<a moz-do-not-send="true"
href="mailto:Proj@lists.maptools.org">Proj@lists.maptools.org</a><br>
<a moz-do-not-send="true"
href="http://lists.maptools.org/mailman/listinfo/proj"
target="_blank">http://lists.maptools.org/mailman/listinfo/proj</a><br>
</blockquote>
</div>
<br>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
Proj mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Proj@lists.maptools.org">Proj@lists.maptools.org</a>
<a class="moz-txt-link-freetext" href="http://lists.maptools.org/mailman/listinfo/proj">http://lists.maptools.org/mailman/listinfo/proj</a></pre>
</blockquote>
<br>
</body>
</html>