How to calculate area under under each category in maxent raster output?

[Syed Amir Manzoor]

Hi,

I've run a SDM using Maxent and I'm interested in calculating the geometry/area of my output. 

I have a question on it and I'm also sharing the different strategies I've been using up till now:

Question: How can I calculate area under each of my habitat suitability class?

To start with, I imported the .asc file (Maxent output) into Arcmap, classified it into 4 habitat classes (Unsuitable, poorly suitable, fairly suitable, highly suitable) and assigned the the value 0,1,2,3 respectively. Now, the first thing I did was to check the attribute table and it showed three columns namely, ObjectID, Value and Count. I reviewed literature on it and as I understood, the count is the number of pixels and multiplying this with the size of the pixel gives you the area. Now, I'm using environmental variables from WorldClim (30-ars-second) which I assume is ~1km in spatial resolution. The total count I get in the attribute table is 4092. What I already know is that the total area of the region I'm studying is nearly 2150 km squared. As I can see, multiplying 4092 with 1 km or anything near to it doesn't get me to the 2150 number. I then realised that it is probably due to the fact that the raster I'm analysing isn't in Projection Coordinate System. I used "Define Projection" options in Arcgis and converted it to British National Grid (since I'm working in UK) but the count in the attribute table remained the same.

Another option I used was to use "zonal geometry as table". When I used this option, I found out that the total area is of all the habitat classes 0.28416. Again, I'm not sure how to interpret this area (given that the raster is in projected coordinate system and the already-known area of the region is 2150 km squared).

Apologies for making it such a long question but I wanted to make sure that your comments will keep all these factors in view. Where am I going wrong in it?

Many thanks,

Amir

[Jamie M. Kass]

In ArcGIS, "Define Projection" tells ArcGIS what the native projection is, so for WorldClim data, this should be WGS 1984, a geographic coordinate system. When you want to project your data to a new coordinate system, you use "Project" to change it. You'll want to project your raster to a projected coordinate system to measure the area of cells in square meters. If your data is projected properly, Zonal Statistics should give you an answer in square meters.

Jamie Kass
PhD Candidate
City College of NY