[postgis-users] Custom CRS for a local survey grid

[Robert Coup]

Hi Mike,

On Sat, May 10, 2008 at 5:03 AM, Mike Toews <mwtoews at sfu.ca> wrote:

> I am working with spatial data that has been collected for the past several
> decades at a site using a custom local coordinate system. I have a few
> points at this site where I know both the local grid and the world
> coordinates. Ideally, I'd like to define a custom srid to transform between
> projections.
>
> I would like to know more about adding new coordinate reference systems to
> the spatial_ref_sys table, however I need to know a few things:
>
>  1. Do I need to define all of: srid, auth_name, auth_srid, srtext,
>     and proj4text fields in the spatial_ref_sys table? Or only a few
>     fields (specify which are required or optional)?


Only the srid and proj4text are actually used. proj4text is passed to proj
for conversion, so anything that works using proj/cs2cs on the console will
work in postgis. WKT/proj4text can be converted via OGR/GDAL so going
between them is easy enough.


> 2. How would I define the projection for a local survey grid? (I may
>     alternately ask this on the proj mail list)
>
> At present, I have a spreadsheet to scale, translate and rotate points to
> NAD83/UTM9N, or I use similar commands in CAD for drawings, but I have much
> more work, and I'd like to make the translation more streamlined.


If you have a definition for the local coordinate system then you should be
able to create the proj4text pretty easily - figure out which parameters you
need and fill in the numbers. By specifying +towgs84 you can add your
scale+translate+rotate as a 7-parameter transform.

http://proj.maptools.org/faq.html
http://proj.maptools.org/gen_parms.html

HTH,

Rob :)
-- 
Koordinates Ltd. (formerly One Track Mind)
PO Box 1604, Shortland St, Auckland 1140, New Zealand
Phone +64-9-966 0433 Fax +64-9-969 0045
Web http://koordinates.com/
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