[postgis-users] Area and scale completely wrong

[Gerry Creager N5JXS]

For all intents and purposes, you can transform to, say, a State Plane 
NAD83, or a UTM NAD83 based SRID and have not really done a datum 
reprojection.

NAD83 and WG84 are essentially the same (minor variations, very minor) 
across Canada, the Continental US, and Central America.

gerry


Nicolas Ribot wrote:
> Johan,
> 
> SRID 4326 seems good to me to represent GPS points (you should check
> your GPS manual to ensure it gives data in the WGS84 Datum).
> As Strk said, you will have to project your data into a cartesian
> coordinate system in order to compute areas and distances.
> 
> Nicolas
> 
> 
> On 11/25/05, Johan Wehtje <genwolf at gmail.com> wrote:
> 
>>Nicholas,
>> I remain a  little unclear about how I determine which SRID to transform
>>to.
>> I have Waypoints downloade from a garmin GPS, and I used WGS84 (SRID 4326)
>>to import the waypoints. I Assume the porblem is that I have the data in Lat
>>Long Decimal Degrees - But I cannot see a corresponding Geographic SRID for
>>WGS 84.
>> ( I am thinking that loading GPS Co-ords into Postgis would have to be good
>>Candidate for an FAQ question).
>> Any ideas?
>> Cheers
>> Johan Wehtje
>>
>>
>>On 11/15/05, Johan Wehtje <genwolf at gmail.com> wrote:
>>
>>>Thanks Nicolas.
>>>I will give that a try
>>>
>>>
>>>
>>>On 11/15/05, Nicolas Ribot < nicky666 at gmail.com> wrote:
>>>
>>>>>Thanks Baren and strk - that was the problem, the file was in decimal
>>>>>degrees.
>>>>>
>>>>> Can these files be used in Postgis ? is it just a matter of finding
>>
>>another
>>
>>>>>SRID, or do I have to convert them sowehow?
>>>>>
>>>>
>>>>Hello Johan
>>>>You can use these files in Postgis, setting the right SRID for them.
>>>>(a geographic system in your case)
>>>>You may also transform them to another reference system by knowing the
>>>>source and target spatial reference systems SRID. (transform()
>>>>function).
>>>>Storing them in lat-long won't allow you to compute precise distances,
>>
>>areas.
>>
>>>>So transforming them to a cartesian reference system is a good option.
>>>>Look in the spatial_ref_sys table to find the SRID corresponding to
>>>>your projection.
>>>>
>>>>Nicolas
>>>>_______________________________________________
>>>>postgis-users mailing list
>>>>postgis-users at postgis.refractions.net
>>>>
>>
>>http://postgis.refractions.net/mailman/listinfo/postgis-users
>>
>>>
>>
>>_______________________________________________
>>postgis-users mailing list
>>postgis-users at postgis.refractions.net
>>http://postgis.refractions.net/mailman/listinfo/postgis-users
>>
>>
>>
> 
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
> 

-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
Page: 979.228.0173
Office: 903A Eller Bldg, TAMU, College Station, TX 77843