<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">Please help me with something potentially important. The documentation for <a href="http://www.gdal.org/gdaldem.html">gdaldem</a> slope says “<span style="font-family: 'Lucida Grande', Verdana, Geneva, Arial, sans-serif; background-color: rgb(255, 255, 255);">If the horizontal unit of the source DEM is degrees (e.g Lat/Long WGS84 projection), you can use scale=111120 if the vertical units are meters</span>” but I don’t think this works very far away from the equator. I think the algorithm assumes that the north-south units are the same as the east-west units, whereas obviously away from the equator <a href="http://www.csgnetwork.com/degreelenllavcalc.html">degrees of longitude become a lot shorter.</a><div><br></div><div>I’m looking at my attempt at avalanche slope mapping, and seem to see east-west slopes showing up with higher slopes than north-south slopes with identically spaced contour intervals.</div><div><br></div><div>Am I right? Or am I seeing things?</div><div><br></div><div>If I’m right, perhaps:</div><div><br></div><div>1) Someone should fix the documentation, so that people don’t rely on incorrect slope calculations for something life-critical such as avalanche slope mapping, and</div><div><br></div><div>2) Someone could tell me a better solution, I think perhaps the best idea is to just resample the grid into a projection based on meters, before applying gdaldem without any scaling factor.</div><div><br></div><div>Thanks for your help. This has been bugging me for sometime, but I finally decided to speak up.</div><div><br></div><div>—</div><div>John Abraham</div><div><a href="mailto:jea@hbaspecto.com">jea@hbaspecto.com</a></div><div><br></div><div><br></div></body></html>