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I received a shapefile containing data from a topographic survey of a farm. The file has a lot of lines containing elevation data. The person who made the survey did it with a GPS set up on a ATV.

When I load the shapefile in QGIS I have the Metadata :

"Geometry type of the features in this layer: Polygon The number of features in this layer: 1942"

I want to create a slope map from this topo survey. So I guess one of the first steps is to create a raster layer from the vector layer. However before that, I think I have to create a GRASS map with the module v.in.ogr.qgis. When I use this module, I have this message :

Building topology for vector map ... Registering primitives... 0 primitives registered 0 vertices registered Nombre de n�uds : 0 Nombre de primitives : 0 Nombre de points : 0 Nombre de lignes :0 Nombre de contours : 0 Nombre de centro�des : 0 Nombre de surfaces : - Nombre d'�les : - Building topology for vector map ... Registering primitives... 0 primitives registered 0 vertices registered Building areas... 0 areas built 0 isles built Attaching islands... Attaching centroids... Nombre de n�uds : 0 Nombre de primitives : 0 Nombre de points : 0 Nombre de lignes :0 Nombre de contours : 0 Nombre de centro�des : 0 Nombre de surfaces : 0 Nombre d'�les : 0 Successfully finished

The module creates a new map name that I can see when I click on "add a GRASS vector layer". However, there is no layer available for this map.

What's wrong? The problem may come from the original shapefile. Bus as I am not a GIS specialist and I am a new QGIS user, I would like to get some help.

I have QGis 1.8.0-Lisboa, Windows 7 and my computer is a French computer (which could explain why there are some French words in the v.in.ogr.qgis message).

Thank you

Best,

Baptiste

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Can you share a sample of your data? Does the projection of your data match the one of your GRASS location? –  Giovanni Manghi Sep 7 '12 at 8:47
    
Hi, thank you for your answer! The projection of my data matches the one of my GRASS location. I can share my data. Can I share them through this forum? –  Baptiste Sep 7 '12 at 15:42
2  
better use something like dropbox, gdrive, ubuntuone, etc. –  Giovanni Manghi Sep 7 '12 at 16:54
    
Here is a link to my shapefile sent with WeTransfer : wetransfer.com/dl/F3XT6Mie/… Thanks again –  Baptiste Sep 7 '12 at 17:15

3 Answers 3

I think your data is in a form that makes it difficult to use. By the look of it, the 'lines' in the shapefile are actually the tracks of the vehicle carrying the GPS kit. As the vehicle traversed the site the GPS was presumably recording lat/lon/elevation triplets for points at preset intervals. What happened to the data after that I can't begin to guess.

If it is possible to obtain the raw data for the survey in the form of (say) a delimited text file containing the lat, lon and elevation for each point this would make your life much easier.

If you are stuck with the data you've got, it might be possible to explode the polygons to points (extract nodes, from under the Vector menu) and then interpolate these to a DEM (under the Raster menu). But I'm not sure about this, the shapefile is so strange.

Added (much) later. There are 10-metre DEMs available for this part of Washington state, perhaps these might be of use? You'll need 2443 and 2444 from here.

Nick.

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Ok, the problem is simple. Your vector is a polygon layer, but it looks like as if it is made of lines, so it means that the "lines" are made of very narrow polygons. When importing vectors into GRASS, v.in.ogr has a "snapping" parameter, that is set by default at a very small value, but that in lat/lon (as your vector is) still is a significant value. So when importing it the polygons borders are snapped and disappear.

To solve the issue just transform your vector to real lines using QGIS "polygon to lines" tool, and then import again into GRASS, it works.

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The min_area parameter (grass.osgeo.org/grass64/manuals/html64_user/v.in.ogr.html) could simply be set to a smaller value...? –  markusN Sep 16 '12 at 9:25

Click the View Output button after the process is completed. See the image below.

enter image description here

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Thank you for your time! I clicked on the View Output button after the process is completed, but then nothing happened. –  Baptiste Sep 7 '12 at 15:45

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