[postgis-users] srid / area

Tue May 16 03:52:03 PDT 2006

Hi Nicolas,

I tried your suggestion,

select area(transform(the_geom, 4148))
 from cdsmriverareaview
WHERE the_geom && expand(geomfromtext('POINT(18.445184 
-33.718208)',4148),0.0002)
AND distance(geomfromtext('POINT(18.445184 -33.718208)',4148),the_geom) 
< 0.0002

Which still results to

3.41666299991106e-06
7.10635000011894e-08
6.35105000257319e-08 *
6.70955000170448e-08
7.29789999820429e-08

I've also tried the other srid's for hartebeeshoek94 as below. My data 
is curently set to 4148

2046;"EPSG";2046;"PROJCS["Hartebeesthoek94 / 
Lo15",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
2047;"EPSG";2047;"PROJCS["Hartebeesthoek94 / 
Lo17",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
2048;"EPSG";2048;"PROJCS["Hartebeesthoek94 / 
Lo19",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
2049;"EPSG";2049;"PROJCS["Hartebeesthoek94 / 
Lo21",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
2050;"EPSG";2050;"PROJCS["Hartebeesthoek94 / 
Lo23",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
2051;"EPSG";2051;"PROJCS["Hartebeesthoek94 / 
Lo25",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
2052;"EPSG";2052;"PROJCS["Hartebeesthoek94 / 
Lo27",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
2053;"EPSG";2053;"PROJCS["Hartebeesthoek94 / 
Lo29",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
2054;"EPSG";2054;"PROJCS["Hartebeesthoek94 / 
Lo31",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
2055;"EPSG";2055;"PROJCS["Hartebeesthoek94 / 
Lo33",GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degr 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs "
4148;"EPSG";4148;"GEOGCS["Hartebeesthoek94",DATUM["Hartebeesthoek94",SPHEROID["WGS 
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6148"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY 
(..)";"+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +no_defs "

Nicolas Ribot wrote:
> Hi Christo,
>
> Setting the SRID to geometric column does not change the data in that
> column, it just tells  the spatial reference system (SRS) of your
> data. Your data are still in degree.
>
> You must use the tranform() function to reproject your data in order
> to compute areas in square meters instead of square degrees.
> Choose a target SRS that preserve areas.
>
> Something like:
> select area(transform(the_geom, <target_srid>)) AS ar
> from cdsmriverareaview
> order by ar
>
> Nicolas
>
> On 5/16/06, Christo Du Preez <christo at mecola.com> wrote:
>> Hi All,
>>
>> I've always got along quite fine importing my data and setting the srid
>> to -1. Now that I need to use the area function I have run the
>> select UpdateGeometrySRID('public', 'cdsmriverarea', 'the_geom', 4148);
>>
>> I'm very inexperienced when it comes to the spacial referencing systems.
>>
>> The data I'm using is Hartebeespoort94/wgs84 in decimal degrees. I used
>> srid 4148 an others but I keep getting the same results, the units of
>> measurement should be in meters, but the result is far from it.
>>
>> eg.
>>
>> select area(the_geom) AS ar
>> from cdsmriverareaview
>> order by ar
>>
>> 6.70955000170448e-08
>>
>> the expected result is roughly 670 sq meters.
>> _______________________________________________
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>> postgis-users at postgis.refractions.net
>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>
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>

-- 
Kind Regards,
Christo Du Preez

Senior Software Engineer
Mecola IT
http://www.mecola.com
+27 [0]83 326 8087