In QGIS 2.18.18 on Windows 10, I have downloaded the NASA file srtm_05_01.tif that is available at http://dwtkns.com/srtm and the airports.shp file available at http://qgis.org/downloads/data/qgis_sample_data.zip. I am using them in an exercise to calculate zonal statistics.
I follow these steps to create my analysis area buffer layer and then calculate the zonal statistics
- Add the airports.shp layer and the srtm_05_01.tif layer to the project
- Vector/Geoprocessing/Tools/Variable Distance Buffer
- Accept the defaults and run
- Save the resulting buffer layer as a shapefile with the CRS set to EPSG:4326 WGS84, the same as the raster file and project
- With the zonal_airport layer created, run Raster/Zonal_Statistics
- Select srtm_05_01 as raster layer and zonal_airport as polygon layer, elev as the prefix
- Select mean, min, max as the stats
When I look at the added attributes in zonal_airport, I get the following (only 4 of the 76 Alaska airports are in the srtm_05_01 sector)
Except for Port Heiden, it first appeared that the elevmax in the results can be mutiplied by 3 to get ELEV original attribute (perhaps unit of measure diffence: feet and meters or yards). But with Port Heiden it is apparent that I may have missed something.
What am I doing wrong?