Jump to content

 
Photo

Slope Calculation from Contour

- - - - -

  • Please log in to reply
7 replies to this topic

#1
cmdrico7812

cmdrico7812

    Key Contributor

  • Validated Member
  • PipPipPip
  • 68 posts
  • United States

I want to do a slope analysis from a contour shapefile that I have. I installed the 60-day trial of the Spatial Analyst extension and I created a GRID from the contour file I have. It is a very detailed contour shapefile with lines every 2 ft; however there are some places that are relatively flat in the area I am analyzing. I've tried many different cell sizes but when I convert the features to a grid, there are areas of the grid that have no value because it is flat thus there are no contour lines there. This really messes with the slope analysis because it doesn't recognize that the elevation in these No Value areas is the same as the contour line around it. Any suggestions on how to fixs this problem? thanks.

#2
Hans van der Maarel

Hans van der Maarel

    CartoTalk Editor-in-Chief

  • Admin
  • PipPipPipPipPipPipPip
  • 3,898 posts
  • Gender:Male
  • Location:The Netherlands
  • Interests:Cartography, GIS, history, popular science, music.
  • Netherlands

I'm not familiar with Spatial Analyst (disclaimer: I've never used it), but wouldn't it give you results all across the dataset, including the flat bits, if you'd generate a TIN for it?
Hans van der Maarel - Cartotalk Editor
Red Geographics
Email: hans@redgeographics.com / Twitter: @redgeographics

#3
cmdrico7812

cmdrico7812

    Key Contributor

  • Validated Member
  • PipPipPip
  • 68 posts
  • United States

You're right, a TIN would give me the result I want. The problem is I don't have the 3D Analyst extension (per another post of mine: I tried installing the 60-day trail of 3D Analyst but the person who was here before me already used up the trial of 3D Analyst) in order to build a TIN. I have the trial version of ET GeoWizard tools but I can't build a TIN with more than 100 features (and my contour file currently has almost 9000 features.

#4
Hans van der Maarel

Hans van der Maarel

    CartoTalk Editor-in-Chief

  • Admin
  • PipPipPipPipPipPipPip
  • 3,898 posts
  • Gender:Male
  • Location:The Netherlands
  • Interests:Cartography, GIS, history, popular science, music.
  • Netherlands

If you email me the contour file at hans-at-redgeographics-dot-com, I can run it through FME and send you a TIN back. Wouldn't get around to it until tomorrow though.
Hans van der Maarel - Cartotalk Editor
Red Geographics
Email: hans@redgeographics.com / Twitter: @redgeographics

#5
frax

frax

    Hall of Fame

  • Associate Admin
  • PipPipPipPipPipPipPip
  • 2,320 posts
  • Gender:Male
  • Location:Stockholm, Sweden
  • Interests:music, hiking, friends, nature, photography, traveling. and maps!
  • Sweden

Hi,
I assume/suspect that you used the contour shape-files as lines, which would be incorrect -- they are actually surfaces/polygons and should denote areas of a certain elevation.
Hugo Ahlenius
Nordpil - custom maps and GIS
http://nordpil.com/
Twitter

#6
rmcculley

rmcculley

    Contributor

  • Validated Member
  • PipPip
  • 38 posts
  • Location:Vermilion, Alberta, Canada
  • Canada

GRASS can do this easily. Using the v.surf.rst command you can create a DEM, Slope, and Aspect grids from your contours. The grid will be in GRASS format which isn't too handy, but then you can use r.out.gdal to output to just about any grid format you can think of.

#7
Charlie Frye

Charlie Frye

    Master Contributor

  • Validated Member
  • PipPipPipPip
  • 112 posts
  • Gender:Male
  • Location:Redlands, CA
  • Interests:Base map design/data model, political/election maps; use of historical maps for modern GIS analysis
  • United States

GRASS can do this easily. Using the v.surf.rst command you can create a DEM, Slope, and Aspect grids from your contours. The grid will be in GRASS format which isn't too handy, but then you can use r.out.gdal to output to just about any grid format you can think of.


The same is also true in ArcInfo, use the TOPOGRID command to convert the contours to a DEM and then use the SLOPE command to generate the slope grid.
Charlie Frye
Chief Cartographer
Software Products Department
ESRI, Redlands, California

#8
CHART

CHART

    Chart

  • Validated Member
  • PipPipPipPipPip
  • 358 posts
  • No Country Selected

There are many tools (software) to convert contours to a GRID in order to do a slope analysis. (e.g. create zones of percentage slopes).

I use Vertical Mapper.

There are also many contour to GRID interpolation methods. To get the a good representative GRID from a set of contours I normally would use the Natural Neighbour with smoothed overshoot method. This approach rounds off hill tops. I would also include spot heights data if available to the coutours before processing.

So I believe the reason there is NO slope value on top of isolated contours is because of the interpolation method used. If the method used does not extrapolate values on hilltops, well your slope analysis is no good.



Hope this helps a bit.
Chart




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users

-->