[GRASS-user] Aspect direction in r.slope.aspect

GRASS GRASS user
1

Hello Grass Users,

I was just wondering why aspect is measured from the east and increased counterclockwise instead of the north and clockwise? It seems to run counter to what is done in other GISs.

Bob

Bob Moskovitz
Research Analyst I
Seismic Hazard Zonation Project
California Geological Survey
http://www.conservation.ca.gov/cgs/shzp

CONFIDENTIALITY NOTICE: This communication is intended only for the use of the individual or entity to which it is addressed. This message contains information from the State of California, California Geological Survey, which may be privileged, confidential and exempt from disclosure under applicable law, including the Electronic Communications Privacy Act. If the reader of this communication is not the intended recipient, you are hereby notified that any dissemination, distribution, or copying of this communication is strictly prohibited.

2

Bob Moskovitz wrote:

I was just wondering why aspect is measured from the east
and increased counterclockwise instead of the north and
clockwise? It seems to run counter to what is done in other
GISs.

It is just measuring theta positive from the x-axis, in the mathematical
sense.

It stays like that for historical reasons, probably been that way for 20+
years. (aka we can only guess at the original logic)

Convert between that and the compass (CW from north) convention with
"90 - angle". Most modules which deal with aspect maps should already
have an option to read the different type.

Hamish

3

On Tue, 2009-01-20 at 12:50 -0800, Hamish wrote:

Bob Moskovitz wrote:
> I was just wondering why aspect is measured from the east
> and increased counterclockwise instead of the north and
> clockwise? It seems to run counter to what is done in other
> GISs.

It is just measuring theta positive from the x-axis, in the mathematical
sense.

It stays like that for historical reasons, probably been that way for 20+
years. (aka we can only guess at the original logic)

Convert between that and the compass (CW from north) convention with
"90 - angle". Most modules which deal with aspect maps should already
have an option to read the different type.

Hamish

This was a small discussion I had today. Supporting an undergraduate
student to work on his thesis, I produced an aspect map for him some
weeks ago. And back then I wrote it *clearly*, in a mail of min towards
him, the "Aspect map values start from EAST and increase
counterclockwise".

Now, he was unsure, so he asked some other guy and he told him... well
you know the rest of the story. Then I pointed to r.slope.aspect manual
and told him to read it. Of course I could re-calculate the values, but
sometimes I prefer to leave thins as they are produced by the module. So
I don't need to take notes about that.

Nikos

4

Thank you for the tip. Btw, I was made aware of this "problem" when a coworker took our landslide hazard workflow and tried to apply it to grass. Everything look great, but when we took another look at the various layers, we were shocked to see how aspect was defined. I know...we should have read the manuals, but everybody knows how aspect is defined. Anyways, we can live with another step in our process, but it would be great if there was a document that shows the differences between GIS systems (such as ESRI, MapInfo, etc), so that we can keep surprises at a minimum. :slight_smile:

Bob Moskovitz
Research Analyst I
Seismic Hazard Evaluation Project
California Geological Survey
http://gmw.consrv.ca.gov/shmp

CONFIDENTIALITY NOTICE: This communication is intended only for the use of the individual or entity to which it is addressed. This message contains information from the State of California, California Geological Survey, which may be privileged, confidential and exempt from disclosure under applicable law, including the Electronic Communications Privacy Act. If the reader of this communication is not the intended recipient, you are hereby notified that any dissemination, distribution, or copying of this communication is strictly prohibited.

-----Original Message-----
From: Hamish [mailto:hamish_b@yahoo.com]
Sent: Tuesday, January 20, 2009 12:50 PM
To: Grass-User (E-mail); Moskovitz, Bob
Subject: Re: [GRASS-user] Aspect direction in r.slope.aspect

Bob Moskovitz wrote:
> I was just wondering why aspect is measured from the east
> and increased counterclockwise instead of the north and
> clockwise? It seems to run counter to what is done in other
> GISs.

It is just measuring theta positive from the x-axis, in the
mathematical
sense.

It stays like that for historical reasons, probably been that
way for 20+
years. (aka we can only guess at the original logic)

Convert between that and the compass (CW from north) convention with
"90 - angle". Most modules which deal with aspect maps should already
have an option to read the different type.

Hamish