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7

Building on Michael's excellent answer, I would recommend using the Con (Spatial Analyst) tool to take a "slice" out of your DEM. The first screenshot shows the parameters you would likely want to use. The second screenshot shows the results of the Con function (as stylized MDOW hillshades) derived from the resulting DEM's.


6

There is two ways of doing this, the best is with Spatial Analyst using Extract by Attributes using the query value < 1.5, this will put NODATA in the areas where the height is greater than 1.5m. If you want to cap the raster to 1.5 so that the values are 1.5 where the raster exceeds 1.5 then use con: Con(InRaster,InRaster,1.5,"value <= 1.5") will ...


5

I've been in LiDAR processing for a couple of years now. The best approach we've found is to classify the suspect water points to something other than ground. Should be easy just classifying based on intensity (near nadir points will have high intensity, whereas turbid water will be close to 0) and laser shots are usually absorbed near shore anyway. ...


5

I've had to map ditches from 1 m LiDAR derived DEMs of agricultural landscapes before. It's certainly a challenging task to come up with a workflow that is suitable. You're ability to successfully extract a ditch network will depend on a number of factors. For example, are you only interested in roadside ditches? If so, are the roads on embankments (as is ...


5

This is a good question, and one that I tend to get asked from time to time. First, as you've pointed out, the equation for TWI = ln(a / tan(B)), where a is the 'specific' catchment area (i.e. the upslope inflowing area normalized for a measure of contour length) and B is the slope gradient, in radians, at the grid cell. As you correctly pointed out TWI will ...


5

Martin is correct that while your workflow will do well for a specific user case, it doesn't account for many of the issues that road embankments create for flowpath modelling using fine-resolution LiDAR data, such as the problems with discontinuous flow in roadside ditches and the effects of minor unmapped culverts (which can alter flowpaths considerably). ...


4

Landsat and Modis are optical sensors, which means that they provide digital numbers of reflected materials that are within the electromagnetic spectrum. These values correspond to the wave length of the corresponding satellite band. To get elevation from just the raw values would be impossible. The only potential means to collect elevation information would ...


3

I was a bit surprised that no one had answered this question yet so I decided to look at how you would perform the task of extracting a watershed from a digitized point along a stream network in QGIS myself. I found that there are about five tools in QGIS for watershed extraction among the various toolboxes. Unfortunately, and perhaps a QGIS expert here can ...


3

You may check out the 'Trace Downslope Flowpaths' tool in Whitebox GAT. It will identify the cells in a DEM that receive flow from upslope target cells. It is however based on the D8 flow algorithm and therefore cannot model flow dispersion, which would yield downslope 'areas' as you are referring to them as. Nonetheless, there is some debate in the ...


3

Chris, Maksim is right, you should be able to solve this problem by mosaicing each of your individual tiles into a single DEM. The problem that you have currently is that each of the individual DEMs has it's own display minimum and maximum values over which the same greyscale palette is being stretched. Consider, for one tile the 256 (perhaps more) grey ...


3

This is the exact problem that I am currently working on for my own research. As a result I have been working on a plugin tool for the open-source GIS Whitebox Geospatial Analysis Tools, called 'Isolate Ground Points'. It will take an input LAS file or point-type shapefile and output only the ground points in a multipoint-type shapefile. It turns out that ...


2

I've run into this same problem in the past. I'm sorry but I don't use QGIS very often so my answer is going to use Whitebox Geospatial Analysis Tools and I'm sure you can find the equivalent tools in QGIS to perform the same task. The basic idea is to interpolate a flood surface based on the elevations from the DEM that coincide with the perimeter of the ...


2

you can use the SRTM DEM as input and create the color relief + hillshade with gdaldem. If the file is too big for your purpose, you can also use ETOPO1.They also provide colored images on this site. If you are looking for a portal, you can visualize several DEM here


2

This is a common ArcGIS problem. I believe as you zoom-in some of the areas that seem not to be there begin appearing as if it was a rendering issue... I think, although it has never been confirmed, that it most often happens when mosaicing tiles from File Geodatabase (FGDB) sources and outputting the mosaic raster to the same or other File Godatabase. ...


2

There are a number of tools in QGIS for generating contours, though most require a raster input rather than vector points. As such, your best bet is to use the Contour Lines From Points tool: Models -> [Example Models] -> Contour lines from points You have to set up the SAGA plugin. You can do it based on the documentation: QGIS Configuring external ...


1

1) I would use the reclassify tool. Set it to twenty 'equal interval' breaks of 5 each. 2) The use this raster ( created in step 1) for rendering. Bring it into ArcGIS and select 20 classes. You could certainly skip step one but it may be nice to have this layer if you plan to use it in future analysis. You could also just do 2 and save as a layer file ...


1

You might as well make a DEM/TIN (using ArcGIS) of the shapefile you have. That will give you the DEM you need to convert to GMT format. After a quick Google search, I found a couple of links.. convert SRTM30 data to grd Shapefile to GMT DEM file Also check the this PDF, mostly after the 15th Page - GMT and Gridded Datasets There is a clear cut ...


1

The answer will depend on the requirements of your specific workflow and application but I can offer you advise on how a drainage network is generally extracted from a digital elevation model (DEM). The key to extracting a drainage network from a DEM is creating a flow accumulation raster, i.e. a raster for which each grid cell contains a value that is ...


1

Creating a mosaic dataset and adding in all your rasters should solve your problem. Create Mosaic Dataset (Data Management) Add Rasters To Mosaic Dataset (Data Management)


1

An option to normalize lidar clouds (and keep it as a point cloud) is Fusion. One will need the command line ClipData together with the switches dtm:file, which is the bare-earth model (DTM) + height. ClipData description says: ...When used in conjunction with a bare-earth surface model, this logic allows for sampling a range of heights above ground ...


1

If I understand the question correctly, the QGIS Raster Calculator will do what you want. For some examples that touch on your problem see: http://spatialgalaxy.net/2012/01/25/using-the-qgis-raster-calculator/ I think you would then clip the output raster using your flood polygon. N.


1

I have had good luck with FUSION's (manual here) GroundFilter command. I've had no problem handling 40 million points (unclassified), so wouldn't expect an issue with 100 million.



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