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Does anyone knows what horizontal unit is used on the table of the "Profile Tool". Vertical is in meter but I don't know about the horizontal axis.

Knowing the length of my profile, I tried to divide that length by the number of extracted elevation but I noticed that the gap between each elevation data seems to change.

At first I have one elevation data each 0.00453 for a hundreds of data. Then its one data per 0.0060 then 0.0105 etc etc.

Would that mean that elevation data are not extracted at equally spaced points ?

Thank you very much in advance !

Note: I am using as raster a .tif DEM and a line built in shapefile.

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Kara, guessing from the related question at gis.stackexchange.com/questions/50108/…, you appear to have multiple accounts. To simplify your life, please merge them by filling out the brief form at gis.stackexchange.com/help/user-merge. –  whuber Feb 14 '13 at 19:00

1 Answer 1

up vote 1 down vote accepted

I have run into this issue in the past, and needed a concrete answer as I was required to document the methodology behind the profile generation for a client. I had found that using an existing shapefile to generate the profile data resulted in highly variable distances between given elevation points, and that when using an interpolated shape (3D analyst - Interpolate Shape - Generate Profile), the distances were more uniform, though not necessarily at an expected interval. I contacted ESRI tech support to clarify my results and they sent me the following response:

To recap our phone conversation, the interval is determined depending on how the vertices were added to the line. The profile graph will read the vertices when creating the graphic. When the Interpolate Shape tool is used the vertices will be added for each cell within the raster, creating a smoother graphic. The interval will be reflected as this interpolation occurs. When the vertices of the original line are used, the graph will be much more coarse and the interval reflect the surface length between vertices.

In summary, when using an existing shapefile, horizontal distance along the X-axis will be in increments determined by the shapefile vertices (units should match the horizontal unit defined for the shapefile). When using the Interpolate Shape approach, horizontal distance should roughly follow a one vertex per DEM cell distribution (i.e. one elevation point will be generated for each DEM pixel intersected by the profile line). In my experience, this interval is additionally affected by the horizontal tolerance of the shapefile, resulting in distance intervals that are close to the DEM cell size, but not exact (e.g. for a 10m resolution DEM, intervals between elevation points will be ~9.996m).

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I touched on this in my answer, but realized I should clarify given the OP's core question: Horizontal units should always match the units of the shapefile's projection. –  JWallace Feb 14 '13 at 18:45
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I'm not sure ESRI tech support has much (authoritative) to say about QGIS :-). –  whuber Feb 14 '13 at 19:01
    
Ahhh...didn't see the QGIS tag. I'll be interested to see if the software-appropriate answer is similar. –  JWallace Feb 14 '13 at 19:03
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Instead of changing the project properties, I would suggest reprojecting your raster to a projection that uses meters, such as UTM (instruction here about half page down: qgis.spatialthoughts.com/2012/04/…). This should address the horizontal unit problem. Unfortunately I don't have a good idea as to why the distance intervals are not equal - have you measured the distance between line vertices to see if they match the distance intervals on the horizontal axis? –  JWallace Feb 15 '13 at 16:10
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Good afternoon JW, I just tried UTM reprojection and it works. Unfortunately the distance interval should not be resolved as I am working on very long distances. UTM refers to a reference parallel, so the more I go away from it, the more the interval will change. But this is my problem now. Thank you very much for your answer ! –  Kara Feb 15 '13 at 19:13

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