[gdal-dev] Re: Slope in gdaldem

[Joaquim Luis]

On 16-12-2011 03:31, Stephen Mather wrote:
> Hmm.  I don't follow?  The revision would be this:
>
> float GDALSlopeHydroAlg (float* afWin, float fDstNoDataValue, void* pData)
>
> {
>     // Hydrologic Slope is the max
>     //  local slope btw center cell and adjacent cells
>
>     const double radiansToDegrees = 180.0 / M_PI;
>     GDALSlopeAlgData* psData = (GDALSlopeAlgData*)pData;
>                 double key;
>
>     float pafLocalMin = afWin[0];
>      float pafLocalMax = afWin[0];
>
>     for ( int k = 1; k < 9; k++)
>     {
>         if (afWin[k] < pafLocalMin)
>         {
>             pafLocalMin=afWin[k];
>         }
>
>         if (afWin[k] > pafLocalMax)
>         {
>             pafLocalMax=afWin[k];
>         }
>     }
>
>     key = afWin[4] - pafLocalMin;
>
>     if (psData->slopeFormat == 1)
>         return (float) (atan(sqrt(key) / (2*psData->scale)) * 
> radiansToDegrees);
>     else
>         return (float) (100*(sqrt(key) / (2*psData->scale)));
> }
>
> But, clearly I'm not understanding something about the window.
>

Steve, there are more things I do not understand either. What is 
'psData->scale'? Is it the grid cell size?
So, if 'pafLocalMin' is on one of the 4 corners the distance is not cell 
size (let's call it DX) but instead sqrt(DX*DX + DX*DX).
And what if data is in geogs? Than DX is clearly different from DY and 
changes with latitude. You need to take that into account to compute the 
slope.

Joaquim