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I used QGIS Raster->Extraction->Contour tool to extract contours at 1m interval from a virtual raster. The virtual raster was built using around 100 tiles in ASC format that represented a 1m DEM.

When zoomed out the contours looked as I expected conforming to the shape of the terrain very well, even picking up minor detail such as dish drains on the side of roads.

However when I zoomed in I noticed that there was numerous missing contours. For example, in the image below showing a ridge, the contours step down at 1m increments from about 101m (bottom of image below) to 96m. However then there is a significant jump to the next contour - some examples are 72m in the top right of the image, to 85m on the left of the image. Looking at the attribute table there also appears to be gaps in data (for instance if I filter display to show contours in the map, there are gaps)

I checked the virtual raster for this area and it looked fine.

I seek advice on how to obtain all the contours as the extracted contours are probably not useful in this state.

enter image description here

Below is a copy of the raster with the contours overlaid. Apologies for the lack of graduation, but you can see different shades of gray showing the terrain. Due to the size of the raster I had alot of trouble changing the styling.

enter image description here

Below is a copy of the settings I used to extract the contours also.

enter image description here

  • Could you provide both an image of the DEM in this area and the exact values you used to create the contourlines? – Erik Jun 12 at 11:29
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    When you look at the attribute table for these data do you see the missing contours between 89 m and 72 m for this area? Are the contours missing from the actual data or just not displaying on your screen? – GBG Jun 12 at 14:26
  • I updated the question with the extra info. Sorry for the delay. – boberdorf Jun 13 at 11:15
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I found a work around on this question. The data tiles I had were generally named for the locality they covered. As the area was large the tiles where spread across three localities. When I processed each of the localities separately and this seemed to work. I am not sure why, because the data source, datum and resolution was the same.

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I think the issue is that the is a limit on the size of SHP files, and changing to a geodatabase seemed to work. I believe the max SHP file size is 2GB.

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