Hi Mike, On Sat, May 10, 2008 at 5:03 AM, Mike Toews <[EMAIL PROTECTED]> 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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