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This question already has an answer here:

I am trying to clip a raster DEM between two elevations, specifically a portion of the contiguous United States in an SRTM 90m DEM.

I would like to create a vector polygon mask that overlies the all parts of the DEM that lie between two elevations; for example, between 1000m and 2000m.

At the moment I have downloaded the SRTM section I need in both ASCII and GeoTIFF formats. I am using QGIS 2.18.15.

marked as duplicate by ahmadhanb, whyzar, tinlyx, Vince, Fezter Feb 5 '18 at 1:42

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    It looks like you're trying to do too much in one step. Why not reclassify to 1=1000-2000m, else=nodata and then convert to vector. It's unclear why you would need to download two versions of the same dataset. – Vince Feb 5 '18 at 1:05
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You want Raster Calculator. Reclassify your raster as follows:

("elevation@1"  < 1000)* 1 + (("elevation@1" >= 1000) AND ("elevation@1" < 2000)) * 2  + ("elevation@1"  >=2000)* 1

Convert the output raster to polygon. Select by expression all features with the value 1 and delete. Remaining features will indicate areas in the desired elevation range.

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