I'm using QGIS 2.0.1. I want to compute areas (in km^2) of all municipalities in Mexico. I understand that I need to convert my map from WGS84 to UTM to get units in km, but I'm not sure how to account for the fact that Mexico spans six UTM zones (from 11N to 16N), or which CRS I should pick from the hundreds available.
Which CRS should I use for this task?
If you want the area of all municipalities of the whole country you can not use UTM. This is because Mexico is located from UTM zones 11 to 16. If you want to calculate areas in km2 you must use INEGI's lambert conformal conic projection. In QGis 2.14 you can find that EPSG 6362 (ITRF92) and 6372 (ITRF2008)have the specific parameters for this projection which are:
+proj=lcc +lat_1=17.5 +lat_2=29.5 +lat_0=12 +lon_0=-102 +x_0=2500000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
As you can see this parameters define the coordinates measuring units in meters. If you want to have the same parameters in another datum you must create a custom CRS (settings/custom CRS). For instance:
INEGI Lambert Conformal Conic, WGS84 datum:
+proj=lcc +lat_1=17.5 +lat_2=29.5 +lat_0=12 +lon_0=-102 +x_0=2500000 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
INEGI lambert Conformal Conic, NAD27 datum:
+proj=lcc +lat_1=17.5 +lat_2=29.5 +lat_0=12 +lon_0=-102 +x_0=2500000 +y_0=0 +ellps=clrk66 +towgs84=-12,130,190,0,0,0,0 +units=m +no_defs
Hope it helps
Gerardo
EPSG:6263 or EPSG:6272. I'll try doing a custom projection.
6362 and 6372 aren't showing up. I'm on Linux Mint 17.3.
+proj=lcc +lat_1=17.5 +lat_2=29.5 +lat_0=12 +lon_0=-102 +x_0=2500000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs, but I still get exactly the same result as using any other CRS (see comments on @afalciano's answer).
I'd avoid using the UTM projection in order to compare areas across six different zones because of the different areal distortions. Instead the Lambert azimuthal equal-area projection (LAEA) is the best option, because it preserves area. For instance, you can use the "US National Atlas Equal Area" (EPSG:2163) or you can define a custom one for Mexico simply changing the coordinates of the natural origin used in the CRS.
EPSG:2163 or WGS 84.
EPSG:2163 or WGS84). So I think QGIS is giving me areas in decimal degrees, because the conversion factor is 106.71^2 (=547.22/0.048057). But this conversion factor varies across municipalities. How do I get QGIS to output the area in km2?
Project Properties | General | Measurements settings are: Ellipsoid: None/ Planimetric; Distance: meters; Area: square km. But I still get the same as before.
EPSG:2163 as CRS, add the new layer to the map and finally do your area calculation dividing the area by 1.000.000 to express it in Kmq, because the unit of measure of EPSG:2163 is meter. Note: the above settings are relative only to measurement, not calculations. Useful link: How to calculate polygon area in QGIS?
Commented
Mar 31, 2017 at 7:59
EPSG:2163 (as well as saving as a new layer with that CRS). When I use Vector | Geometry tools | Export/Add geometry columns, I get area(muni 1002) = 0.048057, but I know that its area in km2 is 547.22. When I use the field calculator and the $area operator I get the same result.
For small areas, as you already said, the best would be choosing the projection of the related UTM zone.
If you are dealing with the whole Country, instead, I think that the best would be choosing the UTM zone for the capital city. For this specific case, I would use the UTM zone 14 North (EPSG:32614), which seems to be the one for which Mexico City falls within.
I would also recommend PolyGeo's option.
OR: You can calculate areas from ellipsoidal coordinates -> Geometry Tools -> Export/add geometry columns -> Calculate using: Ellipsoid
save asto create a new layer with the desired CRS. Do NOT useset layer CRS. Then use the field calculator (attribute table, ctrl-i) with$area.