Aspect and Slope Data

Dear Maxent Group Members,

I used "DEM Surface Tools for ArcGis 10" available from Jenness Enterprises (http://www.jennessent.com/arcgis/surface_area.htm) and extracted slope and aspect rasters from altitude data for ASIA.

SLOPE:

1. The extracted slope (degrees) data ranges from 0 to 53.3299. However, I have read in discussions here that slope should range from 0 to 90 degrees. What is wrong with my method?

2. Can I use asc file derived from this slope data directly in MAXENT or do I need to do some conversions?

ASPECT:

1. The extracted aspect raster shows flat, north (0 - 22.5), northeast (22.5 - 67.5), east (67.5 - 112.5), southeast (112.5 - 157.5), south (157.5 - 202.5), southwest (202.5 - 247.5), west (247.5 - 292.5), northwest (292.5 - 337.5) and north (337.5 - 359.9522) in ArcMap 10.

2. Can I use asc file derived from this aspect data directly in MAXENT or do I need to extract all aspects (North, East, Northwest etc.) separately before running in Maxent. How do I extract different aspects from the raster. I am interested in North, Northeast, Northwest, South, Southwest and Southeast aspects.

3. Do I use the above as continuous or categorical variables?

I would greatly appreciate help.

Thanks.

Rajiv Kalsi

Hi Rajiv,

Although I'm not very experienced with Maxent I believe I can help you
with these questions.

1- There's probably nothing wrong with your slope raster. If your
study area does not have "vertical walls" then you will never reach
90º. In most cases, and depending on the cell size this rarely
happens.

2- Yes you can use the .asc file directly into Maxent. You just need
to make sure that all .asc files have the same extent and cell size.

3- Regarding Aspect you don't need to extract the different aspects.
Just use the .asc file containing all aspect categories directly in
Maxent. After reading some papers I've decided to use Aspect as a
categorical variable.

Cheers,

David

Hi Rajiv, David and all maxent users.

I have been facing the same problem for a long time... aspect as a
categorical or continuous variable?
Sometimes I use it as categorical, sometimes I create a grid and I
calculate the percentage of different aspect classes in each cell,
then I transform each category in a different raster (with values from
0 to 100), using the cells of the grid as pixel, so I can treat them
as continuous.
I think it depends on your purposes, if you are interested in
discriminate the effect and the permutation importance of the single
aspects you should transform them into continuous variables.
David, could you please suggest me some papers you mentioned?

Cheers,
Luca

2012/5/24 DavidBecas <david...@gmail.com>:
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--
Luca Nelli
PhD student
Università degli Studi di Pavia
Dipartimento di Scienze della Terra e dell'Ambiente
Via Ferrata 1,  27100 Pavia
+39 0382 986829
http://www-3.unipv.it/ecoeto/indexstaff.html

This Arc add on, among many other cool topography tools, transforms aspect into a continuos variable from sunny/dry to cold/wet without worrying about degrees.