[postgis-users] Calculate area shperoid

[Brent Wood]

--- Matt Farley <mfarley at sac-yolomvcd.com> wrote:

> Brent,
> 
> I thought Lat/Long was SRID 4269?  What's the difference between 4326 and
> 4269? 
> 

Datum/ellipsoid. As I understand it, 4326 is lat/long with WGS84 ellipsoid &
datum. It's what I use as a default global lat/long projection. I think the
real world difference is pretty trivial between these two.

If your data is not global, then there may well be more suitable projections,
but if you have generic lat/long data which you need to project, then I suggest
4326. (the original question used -1, so I think 4326 is a pretty safe one to
use)

>From memory Paul Ramsey suggested this to me some time ago, so if it's right he
gets the credit, otherwise it's my mistake :-) 


>From my PostGIS spatial_ref_sys table, where you can see a very small
difference in the ellipsoid values (between GRS-80 & WGS84):

nztopo=# select * from spatial_ref_sys where srid=4326 or srid=4269;
 srid | auth_name | auth_srid |                                                
                               srtext                                          
                                      |                proj4text
------+-----------+-----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------
 4326 | EPSG      |      4326 |
GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]
                      | +proj=longlat +ellps=WGS84 +datum=WGS84
 4269 | EPSG      |      4269 |
GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]
| +proj=longlat +ellps=GRS80 +datum=NAD83
(2 rows)


Cheers,

   Brent


> > 
> > 
> > --- luzbrigitte <luzbrigitte at hotmail.com> wrote:
> > 
> > > Hi, anyone knows how can I calculate area to polygons ?  We work with
> > WGS84
> > > system,  srid -1 and PostGis 0.9
> > 
> > If your SRID is -1, then Postgis doesn't know whether the units are
> > degrees/metres etc. If your data is lat/long degrees wgs84, try using a
> SRID
> > like 4326 instead. That allows the transform function to correctly
> reproject
> > your lat/long coords to the equal area projection you want to derive your
> > area
> > result from.
> > 
> > 
> > Brent
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> > 
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