about elevation in dem

iswaryavr

In global Mapper, from online imagery source i downloaded srtm dem (for an area in India) and created a elevation grid data.  The elevation of a particular point is showing as -52m.  In site, the point is an aqueduct over a stream.
what is the negative sign means?
Is the elevation showing is the elevation of aqueduct or stream below it?.
By using mobile topograher, the height at aqueduct is 54m above MSL and altitude as -37.96m above ellipsoid. In toposheet the elevation of the stream is 45m

geomannie

Taken at face value, the negative value indicates that the point is below datum. What is the location of the aqueduct, what datum are you using?

iswaryavr

Datum wgs 84
Location latitude9.04480167, longitude 76.96168112

geomannie

Hi iswaryavr

I have had a look at the location and downloaded the appropriate 3 arc-second SRTM DEM. I cannot replicate your problem. At the location of the bridge/aqueduct, I see an elevtion of 57m, not the -52m that you are reporting. The locality is clearly above sea level. Is it possible that you may have inadvertently altered the elevation values? GM offers the possibility of scaling grids; changing the scale factor from 1 to -1 under the options dialogue could give the negative value you observe.

Regarding your question, is the elevation of the aqueduct or stream below it, it is important to know that the resolution of the 3 arc-second STRM data is about 90m. Thus, the values you see are an average elevation of a cell with a side of about 90m. As a comparison you could try down-loading 1 arc-second elevation SRTM https://earthexplorer.usgs.gov/ or 1 arc-second ASTER https://asterweb.jpl.nasa.gov/gdem.asp, both of which are free. These have an approximate 30m resolution.

iswaryavr

Thanks for your reply. I think i had altered the scale factor. In actual site the elevation difference between aqueduct and stream is about 25m. In 3 arc-second STRM data the elevation of the point is 57m and in 1 arc Aster image the elevation is 48m. what does that difference mean?

geomannie

I am not an expert on digital elevation models, but as far as I am aware no global DEM dataset guarantees 100% accuracy, particularly if you want to reference it to local maps and datums. More significantly, simple maths dictates that if you average an elevation over a 90x90m area or over a 30x30m area, you must get a different answer. In the case of a flat plain, a particular point on a 30m DEM willl have much the same elevation as on a 90m DEM, but in undulating topography it might be quite different.

Neither DEM will resolve the height of the aqueduct. Only a LIDAR survey might resolve that.