I have a broad range of polygons (in vector format and in SpatialPolygonsDataFrame class), some going east-west, others going south-north, while even others span polar regions or very large ranges as mid-to-high/low-to-mid latitudes/longitudes. In addition, as a single polygon can consist of several merged sub-polygons, they do not necessarily have to be aggregated or continuous, but can instead be chopped-up across large ranges. I have until now calculated the area without projecting the data using gArea {rgeos} as follows gArea(SpatialPolygons(spdf@polygons)). Since I am not interested in visualization of any maps but only comparing polygon area and range sizes, I reckoned that the unit (square degrees vs. square meters) did not matter... but now I am not so sure.

So my questions are;

(i) Calculating polygon areas, would I need to project my data using an e.g. equal area projection?

If yes,

(ii) Since I do not care about any map visualizations and my polygons can span very broad ranges making it hard to find a single best projection, can I use the same projection for all polygons e.g. Lambert azimuthal equal-area projection when calculating the area?

  • 1
    Answers to both questions can be found in several threads here, including gis.stackexchange.com/questions/20054 and gis.stackexchange.com/questions/81484. (#1 is definitely yes for otherwise even the relative areas will be grievously in error and #2 is yes because, well, what else would an equal-area projection do?)
    – whuber
    Commented Apr 7, 2014 at 15:43
  • @whuber: Cheers! Sorry for the dup, weird that I manage to miss those threads...
    – jO.
    Commented Apr 7, 2014 at 15:47
  • Searching this site is a little idiosyncratic. I found the dups by searching equal area, ordering the results by votes, and reading through the first page. Googling "equal area polygon site:gis.stackexchange.com" turns up your post (#2 on my hits) followed by some other promising ones, including a duplicate of the "most accurate coordinate system" thread.
    – whuber
    Commented Apr 7, 2014 at 15:51