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I am trying to run a viewshed analysis and I've downloaded DEM data from the UK ordnance survey (Landform-panorama). Since the data is in ASCII format I have created a seamless raster mosaic (and used the projected coordinate system - British National Grid). Now I am trying to display a point layer, with lat/lon coordinates of about 10 sites on top of the DEM raster layer. I have been told that both the raster layer and the point layer needs to have the same coordinate system, but when I use the projected coordinate system on the point layer, my points do not show up. However, if I use the geographic coordinate system on the point layer then the site locations pop up:

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I was wondering whether any one could let me know if this is alright for a viewshed analysis? if not whether anyone could explain in layman's terms what I need to do in order to fix this issue. As always any advice would be greatly appreciated!

  • Because viewshed analysis relies on line-of-sight computations, all that matters for the choice of projection is that linear segments on the map correspond to geodesics on the earth. At the scale any viewshed analysis is conducted (less than a thousand kilometer diameter), almost any projection--even the Equirectangular projection implied by using a geographic coordinate system for plotting--will work fine. Arguably, a Gnomonic projection is an ideal choice because it maps all geodesics to lines. – whuber Nov 29 '13 at 17:32
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Sequence of operations is:

  1. Start ArcMap
  2. Load raster
  3. Confirm that this definitely displaying in meters and is BNG
  4. Create your point, it looks as if you are displaying them as an XY event layer. When you create them make sure you set their projection as WGS84 (lat/long) as this it what they are
  5. They should automatically be re-projected on the fly because the dataframe is BNG. The dataframe is BNG because the first layer you added was your raster which is in BNG.
  6. Right click on point event layer in toc > data > export but make sure you select coordinate system comes from dataframe, this will create a new permanent data set of your points in BNG.
  • I tried it but seems to have hit a hall. There are quite a few WGS 84 (e.g. WGS 1984 Web Mercator Auxiliary Sphere, WGS 1984 Web Mercator). I used the WGS 1984 Web Mercator but it didn't seem to work - the points don't show up on the raster. i checked the dataframe properties, and it is definitely displaying meters and is BNG. why don't my points show up? and which WGS84 should I use? – user19622 Nov 29 '13 at 13:31
  • I also tried the other WGS 84 projection it still didn't work. Any suggestions would be massively appreciated! – user19622 Nov 29 '13 at 13:42
  • Try Geographic Coordinate systems > World > WGS 1984 – Hornbydd Nov 29 '13 at 17:02
  • That worked!! Thank you so very much!So now when i do the viewshed analysis everything will be in the correct projection? – user19622 Nov 29 '13 at 17:55
  • Well it should work unless there is something else causing a problem. Remember to tick this thread if it answered, but only after you've been able to solve the issue. – Hornbydd Nov 29 '13 at 17:56

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