# [postgis-users] SRTM Raster Geometry question

yvecai yvecai at gmail.com
Sun Mar 4 11:30:51 PST 2012

```It seems to me that the SRTM dataset is available in tiles of 1°x1°,
with a 3 arc second resolution, which gives a raster of 1200x 1200
pixels. 3 arc second is ~90 meter, but *exactly* 3 arc second in WGS84.
http://dds.cr.usgs.gov/srtm/version2_1/Documentation/Quickstart.pdf
Yves

Le 04/03/2012 19:36, Ed Linde a écrit :
> Yves, I am not sure I understand what you are saying..sorry. My
> question is basically if the points within a single cell can
> potentially have different elevation values because of interpolation?
> Or do all the points that lie within a cell get one elevation
> value?
>
> On Sun, Mar 4, 2012 at 2:36 PM, yvecai <yvecai at gmail.com
> <mailto:yvecai at gmail.com>> wrote:
>
>     Le 04/03/2012 14:14, Ed Linde a écrit :
>>     Hi All,
>>     Can someone help me understand a concept with the SRTM raster. My
>>     understanding is that the raster can be thought of as a uniform
>>     grid with each cell having dimension =  "90m x 90m" (for SRTM-1
>>     dataset). Now I am just wondering if the elevation values are 4
>>     values at each vertex or boundary point of a cell? or is it one
>>     elevation value for the 90x90 m^2 area covered by the cell? I ask
>>     because if its the first option, then it would mean that every
>>     point that falls in the cell has it's elevation calculated using
>>     some sort of interpolation between the 4 corner points of the
>>     cell to arrive at this point's altitude. Or do several points
>>     that fall into this 90x90 cell get assigned the same elevation value?
>>     Would be great if I could get some help understanding this
>>
>>     Cheers,
>>     Ed
>>
>>
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>     Wouldn't that be a grid of 1°/1201 by 1°/1201 with 1201x1201
>     elevation value?
>     Just a guess.
>     Yves
>
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