# [Proj] Add simple transformations to proj

Jan Hartmann j.l.h.hartmann at uva.nl
Thu Jan 3 04:48:44 EST 2013

```Thanks Jose, I'll give it a try

Jan

On 01/03/2013 02:47 AM, Jose Gonçalves wrote:
> Hello
>
> 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).
>
> 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.
>
> 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.
>
> 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.
> Consider 52.3N as the standard parallel. The following PROJ string
> converts these two points in a way that they have the same easting:
>
>    +ellps=bessel +proj=lcc +lon_0=-16.68875 +lat_0=52.3 +lat_1=52.3
> +lat_2=52.3 +no_defs
>
> The projected coordinates of the first point are (1447575.65 m,
> 205246.75 m). If you add symmetric values as false easting/northing
>
>    +x_0=-1447575.65 +y_0=-205246.75
>
> the PROJ string does what we want. You can also play with k_0 if some
> scale adjustment is still needed.
>
> 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.
>
>
> Regards
>
> Jose Gonçalves
>
>
>
> 2013/1/2 Jan Hartmann <j.l.h.hartmann at uva.nl
> <mailto:j.l.h.hartmann at uva.nl>>
>
>     Hi Frank and all, happy new year!
>
>     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
>
>
>     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:
>
>     xnew = a*xold + b*yold + c
>     ynew = d*xold + e*yold + f
>
>     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.
>
>     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.
>
>     Jan
>
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