What's the difference between SRTM 3sec and SRTM 1sec HG digital elevation models when applying image pair coregisteration?

Other than the spatial resolution difference, which one should I really use while back geocoding? my study area is in the Middle East

both cover the Middle East, and for BackGeocoding the difference is probably minor. But general, SRTM 1Sec has a spatial resolution of 30m and SRTM 3Sec has a spatial resolution of 90 m, so the first is more accurate which is especially contributing to a correct geometric calibration during the Range Doppler Terrain Correction.


Does SRTM 1Sec cover South Asia?

yes, please check here: https://www2.jpl.nasa.gov/srtm/coverage.html

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Just a small comment.
One may try Radiometric Terrain Corrected products from ASF where for some areas there are 12.5 DEM included in the bundle. Not sure of there will be improvements, but it is worth test the option.

the pixel spacing is 12.5 m, but most of the products originate from SRTM.
As shown in their table, only US territories and Alaska are based on NED which is below 30m. The rest is just resampled but without increase of quality.

source: https://asf.alaska.edu/data-sets/derived-data-sets/alos-palsar-rtc/alos-palsar-radiometric-terrain-correction/

Poor resolution DEMs are perfectly fine for co-registration - using a better DEM brings no benefit.

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