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Thread: Innovyze Tips and Tricks:Elevation Extraction using Contours in InfoWater

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    Lightbulb Innovyze Tips and Tricks:Elevation Extraction using Contours in InfoWater

    How Elevation Extraction from Contours works in InfoWater and InfoWater SA:

    In InfoWater elevations are best assigned using a raster, but because elevation data in the past was often in contour files we still maintain the ability to extract elevation data from contour data. While elevations can be extracted from contour lines, users sometimes note the elevation extractions can take a long time to be assigned and that the elevations assigned do not always fall within the contour lines the junctions falls between. This occurs because when assigning elevations from contours the software is actually converting the contour lines to a basic raster and then extracting the elevations from that surface. Because the conversion is completed in a simple conversion and this raster is often has a large pixel value this can create unexpected differences between the contour data and the raster used to assign elevations. Since rasters assign one elevation to each pixel this causes what appear to be inaccuracies in the elevation extraction when it has more to do with the raster properties it used to create the pixel size which is the Resolution(Map Units) field which is what the pixel size is based on. Since the default Resolution is assigned by ArcGIS so that the entire contour area is covered and the pixel default size is often large this means that assigning elevations from contours may not always assign elevations from contours as accurately as desired. See the examples below:

    Example: Notice how the Resolution(Map Units) field Pixel Value has to average elevations within each pixel. Note how large the pixel size is relative to the contour lines:
    (click if need larger image)
    Basic Raster Pixel.png

    However, if you try to change the Default Resolution value used the it will create holes in the Raster as it will create only a buffer zone around each contour: This is why the default units are used in the basic conversion and why the resolution value assigned is not really changeable to try and increase the accuracy.
    (click if need larger image)
    Raster -holes.png

    One is often better off first finding if you have an elevation surface you can use from the client (these are so readily found nowadays it is getting almost rare not to have one since most aerial photography these days includes LIDAR which is often used to create this data), but if you are in an area and only have access to contour data, you may not have that option.

    If you find you only have contour data available then it is often advisable to convert the contours to a raster yourself (or have your GIS group do it) so that you have the raster to use, and that you can see how well the raster you make compares to the contours.

    If you wish to see the basic raster created by the software use this tool: Arctoolbox-> Conversion Tools -> To Raster -> Polyline to Raster tool. This is included with any ArcGIS license and can be used to see a basic raster created by the software. Even just creating this raster will allow you to see what the tool is using and use this raster to assign elevations more quickly as each time you assign elevations in the software it will recreate the raster and this takes time.

    (click if need larger image)
    basic raster conversion.jpg

    If you have advanced tools you can convert the contour lines to a Raster using more advanced methods which will take longer, but will result in a more accurate elevation surface to sue. We have been able to make fairly decent raster’s using the Arctoolbox-> 3D Analyst Tools -> Raster Interpolation -> Topo to Raster tool, but making Rasters of large contour areas can take a long time to generate and you generally need a 3D analyst , or ArcInfo (Advanced) type license to have access to these tools. This tool allows you to interpolate between contours to get the most accurate raster, which is much better than the base convert contours to raster tool which if you use map units less than what they recommend will create "holes" in the pixels as you move away from the contour lines.
    (click if need larger image)
    Advanced Controur to Raster Conversion.jpg

    We have had decent success creating rasters using 20 ft “cells or pixels” but the smaller the pixel the longer it can take to convert a set of contours. Somewhere in the 20-40 ft range seems to be fairly reasonable in how the rasters look but as noted it takes a long time and a lot of computing power to convert these so it is best to have your GIS staff do it for you. Please note larger contours line files could take several days to process using this advanced procedure, so use caution when choosing the pixel size.

    Example: What the higher quality Raster conversion of the contours looks like: Notice the shape of the raster contours more closely follows the contour lines and the pixel size is much smaller. This small area was converted using a 10 ft pixel value:
    (click if need larger image)
    Advanced Raster Example.png

    This should better help you understand how the model extracts elevations from contours and how one can use conversion tools to improve the process.

    Incidentally this is why the extraction by Raster tends to also be way faster than the contour extraction because each time you run a contour extraction it is going to create a basic raster and this is the time consuming step in the process.

    Often times the extraction is not too bad from a contour file, but this is why you can get elevation values outside the contour lines because it does not really use the contours themselves but the basic raster created to assign the elevations.

    If you convert your contours to a raster yourself you can verify the quality of the conversion and get a smaller pixel value which leads to greater accuracy in the elevation assignment.


    Please contact us at support@innovyze.com if you have any further questions in this matter.

    Patrick Moore
    Last edited by Patrick Moore; August 23, 2018 at 12:19 PM.

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