Vanama, can you provide a URL to the place where you downloaded this file at? Typically DEMs won't have negative values unless they're actually under sea level. I can take a look at the file and see if it appears to have negative values for me as well. 

To create an XYZ file, you can do this with Esri's ArcGIS Desktop software using the Spatial Analyst extension. http://www.exprodat.com/blogs/blog_Converting-ArcGIS-Rasters-to-XYZ-ASCII-Files.htm

-Andy

There are two type of elevation; assume and real which is based on MSL. Both can be negative or positive, if based on MSL it means lower MSL but if based on assume BM it can be higher or lower the MSL.

Dear andrew

Please find the location of my study area and the Dem file obtained from Bhuvan.

https://www.google.co.in/maps/@19.0506901,72.9011199,12z?hl=en

Vanama,

That's interesting. I'm not sure why there are negative values in your DEM. If I were you, I would try to source another DEM from a different place. 

If you're looking for a DEM, the United States Geological Survey has a website that has some world-wide DEMs on it. The highest resolution for the DEM they provide for your project area is 7.5 arc seconds. If that's sufficient, you can download it here: http://earthexplorer.usgs.gov/ (it is the GMTED2010 layer on the map). 

Hope that helps,

-Andy

Oh tanks

  If the elevation values is negative，you can Multiplied by -1 use Raster calculator of Arcmap，it maybe useful.

Dear beleit bai,

If we multiply by -1 then the negative values become positive and the positive values become negative which is not of use.

In my view，the data should be reclassified（reclassify of 3D analysis ） then we can multiply by -1 for he data which the elevation values are negative.I have no idea whether it is feasible。

You could easily change the negative-valued cells to positive values with a conditional statement, but my only question would be whether this is accurate any more? The main question is, are the negative value cells actually correct, with a sign change? Or, are they completely wrong and therefore multiplying by a -1 will still not give you a correct cell value because it is fundamentally wrong.

You may add a compensating +ve value to all pixels. The compensating value may be found by comparing elevations of known points.

You may add a compensating +ve value to all pixels. The compensating value may be found by comparing elevations of known points.

you should check out the documentation for CartoDEM v3. I think they've added -255m to all the coastline so you can make a raster calculation and add 255 to each cell value and your coastline will be at 0m. As to why they have done that, I'm not entirely sure!!