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I am looking at MapX and MapY coordinates. The docs say they are "truncated State Plane coordinates: NAD 1983 State Plane Louisiana South FIPS 1702 (US Feet)."

I am trying to reproject into a common lat/long like wgs-84 by using PostGIS so I need the srid for this projection.

I found the spatial reference page for the state plane coordinates http://spatialreference.org/ref/esri/102682/ -- but it does not list the SRID.

I found this page of common SRIDs -- but if I plug in the SRID the projection does not output correct lat/longs.

How can I find an authoritative SRID?

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Is it EPSG:3452? I'm not familiar with that projection, but the SRID 3452 one uses US ft. –  SaultDon Mar 30 at 0:40

2 Answers 2

up vote 2 down vote accepted

The proj definition for EPSG:102682 is

+proj=lcc +lat_1=29.3 +lat_2=30.7 +lat_0=28.5 +lon_0=-91.33333333333333 +x_0=1000000 +y_0=0 +datum=NAD83 +units=us-ft +no_defs

and for EPSG:3452 it is

+proj=lcc +lat_1=30.7 +lat_2=29.3 +lat_0=28.5 +lon_0=-91.33333333333333 +x_0=999999.9999898402 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs

So lat_1 and lat_2 are swapped (which makes no difference) and x_0 is off by less than one millimeter.

I don't see any reason not to call them identical.

For official EPSG projections, the SRID in postgis is identical with the EPSG code. But 102682 is from ESRI, that's why you will not find it in the official EPSG database. PROJ/GDAL/QGIS call it EPSG:102682, but it should rather be ESRI:102682 (as spatialreference.org calls it).

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At the very bottom of that link to common SRIDs is a line with a link to the EPSG registry where you can search for more SRIDs. Putting in Louisiana South comes up with a few options, the one SaultDon mentioned (EPSG:3452) being the closest match (ie, NAD83, no HARN, and using feet).

However, the word 'truncated' gives me pause. A quick search led to this thread on surveyconnect where they debate the merits of state plane, and in it they talk about truncated being 'not true state plane' but rather essentially a custom coordinate grid. It might be they just divided off some decimal places to make for shorter numbers; or they may have applied some kind of additional correction as well. If there is no further documentation on how the coordinates were truncated, or you cannot reverse the process to get true state plane, no SRID will do you any good.

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Yes, in addition to being in stateplane (EPSG:3452), the coordinates have probably been re-scaled in some way. If you can take a single known point in your map, and manually figure out what the lon/lat coordinates are, then project those coordinates INTO EPSG:3452, you can compare the coordinates and see if you can figure out the offset they've taken out of the coordinates. –  Paul Ramsey Mar 30 at 7:02
    
Oh, and once you've figured out the custom offset, you can create your own spatial_ref_sys entry, starting from the EPSG:3452 definition, and then altering the x_0 and y_0 parameters to correct for the custom offset. –  Paul Ramsey Mar 30 at 7:04
    
@ChrisW I don't see any mention of SRID in the ESPG registry. How do I go from ESPG to SRID? –  akh2103 Mar 30 at 14:29
    
See my extended answer. –  AndreJ Mar 30 at 18:06
    
@akh2103 Andre has already explained a little below, but essentially in that registry the 'code' column is the SRID. You might take a look at the Wikipedia entry regarding SRIDs: en.wikipedia.org/wiki/SRID It basically explains that SRIDs can be created by vendors or an authority and that, as Andre points out below, who created the number is/should be part of the designation as much as the number itself. –  Chris W Mar 30 at 18:41

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