[Qgis-user] Slope and area/length calculations with Geographic CRS

[Gerardo Jimenez]

Try working with specific value for the latitude you are working. Check this

http://www.csgnetwork.com/degreelenllavcalc.html

As I understand Gdal uses the lenght of a degree of latitude in meters of feet, so this can help to solve the problem.

Gerardo Jiménez Delgado
Instituto de Investigaciones Antropológicas
Universidad Nacional Autónoma de México
Ciudad Universitaria s/n
Coyoacan 04510
Mexico City
Mexico

56 22 95 16

Date: Wed, 11 Jan 2012 20:30:30 +0530
Subject: Re: [Qgis-user] Slope and area/length calculations with Geographic CRS
From: harry.om2000 at gmail.com
To: gejst5 at hotmail.com

Thanks! it is working well.

For feet it seems that slope is less on each pixel with the scale value 370400 so we can increase the value and check again,
but for meters with scale value of 111120, it is working well and equal to the slope found with Projected CRS on each pixel.


Thanks Again!

Regards

On Wed, Jan 11, 2012 at 7:41 PM, Gerardo Jimenez <gejst5 at hotmail.com> wrote:





For slope calculations with lat lon coordinates and elevation units in meters or feet you have to be aware to set the proper "scale (ratio of vertical unit to horizal units)" value.  Gdal help recomends using 111120 if vertical units are in meters and 370400 if they are in feet. 

Seehttp://www.gdal.org/gdaldem.html
Hope it helps

Gerardo Jiménez Delgado
Instituto de Investigaciones Antropológicas

Universidad Nacional Autónoma de México
Ciudad Universitaria s/n
Coyoacan 04510
Mexico City
Mexico

56 22 95 16

Date: Wed, 11 Jan 2012 19:18:51 +0530
From: harry.om2000 at gmail.com
To: Qgis-user at lists.osgeo.org

CC: 
Subject: [Qgis-user] Slope and area/length calculations with Geographic CRS

Respected All,

When we have to calculate area/length in polygon/line  vector layers we have to convert Geographic CRS of the layer in Projected CRS, and in raster DEM for slope calculations the same is to be done.



I am providing continuous free practical trainings to the Government officials in India of 5-6 days on Application of Desktop QGIS in Watershed/Rural Development schemes and in this duration I don't want to put the trainees deep in the matter of Coordinate Reference System and want to do everything in GCS-WGS1984. Can there be any solution presently or in future for calculating area/length in vector and slope etc. in DEM with geographic coordinate system. 



Regards

Harish

-- 
Harish Kumar Solanki, Assistant Professor
National Institute of Rural Development (NIRD), MoRD, GoI 
Jaipur, INDIA- 302006


Email- hksolanki at nird.gov.in, harry.om2000 at gmail.com, 


Tele-Fax-0141-2209904, Mo.- 09414291732, 09214446732
Website-  http://www.nird.org.in, http://watershed.rajasthan.gov.in/
LinkedIn: www.linkedIn.com/in/hksolanki





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-- 
Harish Kumar Solanki, Assistant Professor
National Institute of Rural Development (NIRD), MoRD, GoI 
Jaipur, INDIA- 302006


Email- hksolanki at nird.gov.in, harry.om2000 at gmail.com, 

Tele-Fax-0141-2209904, Mo.- 09414291732, 09214446732
Website-  http://www.nird.org.in, http://watershed.rajasthan.gov.in/
LinkedIn: www.linkedIn.com/in/hksolanki


 		 	   		  
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