[postgis-users] SRTM Raster Geometry question

Puneet Kishor punk.kish at gmail.com
Sun Mar 4 21:54:53 PST 2012

```On Mar 4, 2012, at 4:43 PM, Puneet Kishor wrote:

> Yves is absolutely correct. Each pixel that is "no data" is a separate value.

whereby, I meant that each pixel that is NOT a "no data" is one elevation value. `gdallocationinfo` can happily query each pixel (or lng/lat) quite efficiently.

>
>
> On Mar 4, 2012, at 3:25 PM, yvecai wrote:
>
>> I think one pixel= one elevation value, and :
>> "File names refer to the latitude and longitude of the lower left corner of
>> the tile - e.g. N37W105 has its lower left corner at 37 degrees north
>> latitude and 105 degrees west longitude. To be more exact, these
>> coordinates refer to the geometric center of the lower left pixel, which in
>> the case of SRTM3 data will be about 90 meters in extent."
>>
>> All the doc is here:
>> http://dds.cr.usgs.gov/srtm/version2_1/Documentation/
>> ;-)
>> Yves
>>
>> Le 04/03/2012 20:53, Mr. Puneet Kishor a écrit :
>>> On Mar 4, 2012, at 1:39 PM, Ed Linde wrote:
>>>
>>>> Thanks Yves, but my question still remains though. So in this raster of
>>>> 1200 x 1200 pixels, does each pixel get assigned an elevation in meters of
>>>> "x" meters? Or do the four corners of each pixel have different elevation
>>>> values of a,b,c and d meters each?
>>>> ..
>>> I am guessing (albeit, based on my fiddling with SRTM a long time ago) that it is an ave value of "x" meters assigned to all the pixels in the tile. In essence, a 1200 pixel-square tile is behaving like a single large pixel with a value of "x" meters.
>>>
>>>
>>> Should be easy to confirm it by querying a few random pixels in a tile.
>>>
>>>
>>>
>>>
>>> --
>>> Puneet Kishor
>>>
>

```