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i downloaded the new (dutch hight data) AHN files, and was trying to perform a viewshed analysis at some point near UTRECHT.

The plugin (grass plugin) works ok, i tried it before with a dem map.

But when i use the AHN file it shows not the correct visibility area.

so my question is

  1. Does someone know how to setup the AHN files, for using in a viewshed analysis
  2. Does someone know how to make the real height information visible on a map

elevation data "utrechtse heuvelrug"

This a part of the tutorial from the book. As seen the part done by the plugin shows much less data

  • Which AHN set do you use? The AHN2 seems to have holes at buildings. – AndreJ Jul 3 '15 at 11:37
  • Hi AndreJ, I used the AHN25. Is it possible that qgis only colors the pixels that has the same height as the observer height? and that every other pixel (Higher and lower) is ignored? geodata.nationaalgeoregister.nl/ahn1/atom/ahn1_25m.xml – BearHunt Jul 3 '15 at 14:39
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    The plugin works for me with SRTM and with AHN data, but giving different results. Keep in mind that all layers should share the same CRS, and the AHN elevation should better be scaled from cm to meters. You should clip the AHN DEM to your area of interest to bypass memory error. If it does not work for you, add a screenshot with the plugin parameters you have set. – AndreJ Jul 3 '15 at 16:40
  • BTW I used the qgis plugin, not GRASS. – AndreJ Jul 4 '15 at 4:57
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To get a coloured picture of the AHN DEM, add a NODATA value of -2147483647 in the Transparency tab.

In the Style tab, load the Extrema ( not the default 2% to 98%) and select one channel pseudo colour. With the Spectral colour ramp [x] inverted, the output looks like this:

enter image description here

For the viewshed, I clipped the data to my area of interest and converted the elevation data from cm to meters with the raster calculator. For the Observation point, I randomly chose (137481 455573) in RD New EPSG:28992 (same as the AHN raster has).

The parameters for the analysis plugin are:

enter image description here

Setting the outputs layer transparency to 1, the result looks like

enter image description here

Increasing the observer height to 300m, the picture is even more complete, with some shadows behind the hills:

enter image description here

  • With "load the Extrema" you mean "min / max" value radio button? – BearHunt Jul 3 '15 at 23:14
  • Yes, and actual instead of estimate even drags it further out. – AndreJ Jul 4 '15 at 4:56
  • Ok, ive got it. the values in the "layer bar" are those the height data in cm? – BearHunt Jul 4 '15 at 12:18
  • Strange, i used the qgis plugin and the grass gis plugin. both are generating different outputs. i have the book "Mastering QGIS" from Kurt Menke. In chapter 7 is the viewshed described. I was now trying to do the same for the netherlands ("utrechtse heuvelrug" area) but when i look at the generated output, i have the feeling that it is not the whole thing. It seems that the output shows to less visible points. is that becouse of the filetype? – BearHunt Jul 4 '15 at 12:46
  • Compared to SRTM, the field values should indeed be heights in cm. It might help to recalculate them to meters using the field calculator. The results look different for me. In your example, the observer height is 1.6 (meters), while the raster data is about 6000 (cm). Apart from this plugin, GRASS and SAGA can also do viewshed analysis. github.com/zoran14/viewshed and hub.qgis.org/projects/viewshed/wiki give some hints on the usage. – AndreJ Jul 4 '15 at 15:34

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