[postgis-users] question on SRID setting

[Michael O'Sullivan]

Hi,

I am very new to the world of postgis (day 1) - and would like some 
clarification on how the spatial_ref_sys table and SRID values in other 
tables of data I load work.

As background I have compiled postgresql and postgis, and loaded the 
spatial_ref_sys.sql
data that comes with the postgis distribution.

As an exercise I have then downloaded some shapefile data for US ports from

http://www.bts.gov/gis/download_sites/ntad02/newusdownloadform.html


It had a web page with metadata as below

> /Spatial_Reference_Information:/
>
> /Horizontal_Coordinate_System_Definition:/
>
> /Geographic:/
>
> /Latitude_Resolution:/ 0.000001
> /Longitude_Resolution:/ 0.000001
> /Geographic_Coordinate_Units:/ Decimal degrees
>
> /Geodetic_Model:/
>
>     /Horizontal_Datum_Name:/ North American Datum of 1983
>     /Ellipsoid_Name:/ Geodetic Reference System 80
>     /Semi-major_Axis:/ 6378137.000000
>     /Denominator_of_Flattening_Ratio:/ 298.257222
>
> /Vertical_Coordinate_System_Definition:/
>
> /Altitude_System_Definition:/
>
> /Altitude_Resolution:/ 1.000000
> /Altitude_Encoding_Method:/
> Explicit elevation coordinate included with horizontal coordinates
>

After looking through the spatial_ref_sys.sql file I decided on using 
the following SRID
---
--- NAD83
---
INSERT INTO "spatial_ref_sys" 
("srid","auth_name","auth_srid","srtext","proj4text") VALUES 
(4269,'EPSG',4269,'GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Gre
enwich",0],UNIT["Degree",0.017453292519943295]]','+proj=longlat 
+ellps=GRS80 +datum=NAD83 ');
---

Is this the correct SRID entry?

There are a number of other "NAD83"  entries, (e.g. NAD83 / UTM zone 5N) 
these seem to be subsets of the NAD83 entry. I notice these have 
PROJECTION entries which seem to be defining these subsets from what I 
can make out.

As a rule - if I am downloading data (usually in shapefile format I 
guess?) I will have to obtain the relevant metadata so I can work out 
the SRID to use?

As I said this is all very new to me so please excuse me if this 
question is rather obvious. I have been reading different bits and 
pieces of the install doco today and am still trying to put it all 
together. Feel free to point me to any  links which may be relevant.

thanks,

Michael