Irricad

Hi, which data will Irricad use to calculate the DEM if I am using imported gps data for field boundaries and downloaded lidar data for elevation? Will the gps points (which contain elevation) interfere with the imported lidar data (which is in dxf format)? The lidar data is imported as a contour line with no individual spot heights. Because the gps data is not perfectly match the lidar data, it seems to interfere. Is there a way to delete the gps data afterwards so that they don't interfere? Any help or insight is appreciated. Thanks

Corby

Hi Corby

Irricad only uses items for interpolating the values for the DEM that you specify with the "Import Contours" (also used to import spot heights) or "Convert to Elevations" commands. These processes turn the specified items into Irricad "Contours" or "Spot Heights".

Consequently if you import your GPS data using the standard "Import" command any elevations in this data will not be used when calculating the DEM.

Phil

Is there a way to clear the DEM without having to start all over? It looks like I now have gps data and lidar data being calculated into the DEM and they aren't matching up correctly.

Also, how does Irricad use elevation data that is already in the form of contour lines instead of spot heights? I am currently calculating contour lines from lidar data using another software and then importing this as a imported elvation into Irricad, but often I see small problems with the DEM when doing this.

corby

Running "Interpolate Elevations" from the "Design" menu will re-create the DEM but obviously this will take account of any existing IRRICAD contours or spot heights. As previously mentioned unless your GPS data has been imported via the "Import Contours" function it cannot effect the DEM. If it was imported as contours then these entities would need to be removed before running "Interpolate Elevations".

It depends on what DEM option your are using (Design/Design Parameters/Analysis Parameters) but assuming it is the default "ABOS" then spot heights are actually taken from the contours at the interval specified by the "DEM Resolution" field (the settings are described in the manual under "Tools and Command Reference/Design/Design Parameters/Analysis Parameters"). Making this value smaller potentially increases the accuracy but will mean the interpolation takes longer to run.

The two most common causes of differences between the DEM and the base data are:-

1) Incorrect data points - if these specify sudden changes in elevation it may not be possible for the DEM to model these accurately.

2) The DEM Grid Size being too small - setting this to 0 will choose a suitable grid size but this may result in a large number of grid points and consequently slow interpolation.

If you are still having problems you may want to contact your local IRRICAD support (irricad@nelsonirrigation.com) or you are welcome to send us the design (support@irricad.com) and I will take a look at it.

Phil

Thanks for the help.