qgisrooftop-buildingunits
I have a shapefile with building footprint outlines. I am trying to identify buildings that are a certain size in meters squared. The shapefile that I has, has a SHAPE_Area column, but I cannot tell what unit it is.
How can I tell what the units currently are?
The CRS for this layer is EPSG:2260, NAD83 /New York East (ftUS).
Map units are defined by the source layer's projection. You should check your original source file, confirm that it is actually in UTM (and showing projected coordinates in metres, as in the example below) and not simply set as UTM (which can be done falsely).
Add a point layer:
Confirm it's in UTM (also note the displayed coordinates in the above screenshot, '703656,9798666', which are clearly not degrees):
Rasterize it - in this case, looking for a 2500x2500m pixel:
Output grid is 2500m pixels:
I suggest to use EPSG:25832 ETRS89 / UTM zone 32N, because the Gauss-Krueger DHDN CRS zones are based on the old bessel ellipsoid, and you have to apply a datum shift that is not constant over the whole of Germany.
ETRS89 has no datum shift to EPSG:4326 WGS84 which you can get from GPS receivers, and all official cadastral information has beeen moved to ETRS89 by now.
You could set up a local transverse mercator projection on a center point of your area of interest as well, but that would gain only little improvement on accuracy against UTM.
By the way, EPSG:3857 has no real meters as units, they match only at the equator.
Best Answer
I created a building footprint for one aleatory selection on map at East Hampton (New York). The CRS for this layer is EPSG:2260, NAD83 /New York East (ftUS). Afterward, I created 'SHAPE_area' field and calculated its corresponding area by using field calculator (geometry section). Despite CRS is expressed in feet, area was determined in square meters by default. So, it was also calculated area in square feet ('area_ft2' field) multiplying 'SHAPE_area' field by 10.764.
At following image it can be observed approximate area determined with Measure Area Tool of QGIS (square feet). In your case, compare result with value in 'SHAPE_area' field for one arbitrary record. If it coincides, units are in square feet. So, use 10.7639 as dividing conversion factor to get your new field in square meters.
If it doesn't coincide, use Measure Area Tool of QGIS in square meters for verifying field area values.