# Tag Info

17

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 ...

14

When converting a LiDAR dataset to a DEM, you are taking a set of discrete data points and converting them into a single, continuous dataset. Let's say that your .las file contains X (latitude), Y (longitude) and Z (elevation) values with an average resolution of ~1 meters. The resolution here is really important- we're only talking about an average and so ...

13

Choosing the right algorithm for hydrologically correcting a DEM really depends on your particular application. DEM size is certainly an issue, as you discovered. If you have a massive DEM, there are only a few algorithms that will work for your application. Another important consideration is whether or not all of the topographic depressions in your DEM are ...

13

While Jacub is correct that a stream ordering technique is your best bet for being able to represent stream magnitude or discharge, since the position in the network is a surrogate for these two stream characteristics, I would argue that you want to avoid using Horton-Strahler stream order for this purpose. It is seriously deficient in these applications, ...

12

Have you consider to use GRASS GIS analysis? I have expirience that GRASS algorithms have very good accurance on hydrology analysis. For example, I want to generate something like drainage network on DTM with resolution 5x5m. I had compared tools from ArcMap (including ArcHydro Tools) and you can view the result on first picture (red lines). Then I tried to ...

11

There is a difference, and I recommend the typology presented by Lindsay (2015) be used. Lindsay (2015) presents a typology which defines a pit as a single cell in a DEM whose elevation is below that of the surrounding cells and a depression as a region of cells which drain inwards to a pit. This is consistent with the definitions used by O'Callaghan and ...

10

The Flow Direction tool in Spatial Analyst only supports the D8 flow direction model, as stated in the How flow direction works page: There are eight valid output directions relating to the eight adjacent cells into which flow could travel. This approach is commonly referred to as an eight-direction (D8) flow model and follows an approach presented in ...

10

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). ...

10

You can classify the streams using, among others, the Strahler Stream Ordering method. In the Strahler method, all links without any tributaries are assigned an order of 1 and are referred to as first order. The stream order increases when streams of the same order intersect. Therefore, the intersection of two first-order links will create a ...

9

In spatial hydrology, DEM-based flow accumulation operations are typically static. That is, they represent a steady-state condition of the discharge of surface and near surface water passing through a point. Flow accumulation grids are actually accumulating contributing area downslope, i.e. areas not volumes. The accumulated cells that you described is ...

9

I'm afraid that when I wrote Whitebox GAT, I never really anticipated that there would be a demand to be able to run the Whitebox plugin tools from outside the user interface. I knew that there would be a need for automating workflows and that's why I wrote the scripting capabilities into it, such that each tool can be called internally from a Python, ...

8

As @Hornbydd pointed it is network searching problem. I suggest the following workflow: find source points of the streams sort them in descending order by flow length In the picture below 139 green points are sources labelled by their sequential order and elevation, e.g. remotest point (1/435). There are 2 possible paths from here: Trace and dissolve ...

8

You need to adjust the "threshold" parameter, which is Minimum flow accumulation for streams in the Processing GUI. With this parameter you determine what is the minimum drainage area for each stream reach. THe units are pixels. So in the above image, with Minimum flow accumulation=1 you will be creating a very dense stream network, where each and every ...

7

This looks like a valid workflow for this task. However, depending on the level of accuracy that you work with, you will miss out on things like pipes under roads allowing for ditches and small creeks to flow past the road. These would probably require some field work to identify though. You don't mention which software you use, but for ArcMap point (6) ...

7

Ok here is the image Blue are the bridges from OSM. The DEM is in grey-scale with buildings (I want buildings) I selected the intersect roads that cross the bridges. Buffered the selected roads by the approximate width of a road. Ran 'Zonal Minimum' on the buffer. Now you have the zonal minimum raster (red) with the low values. The I just used CON to ...

7

It is an error. Your link and presumably image source is the 10.0 help. The error is still present in the 10.1 help. However in the 10.2 help, it has been corrected to show only a 1. It's also worth noting that if you work through the math matrix as whuber has done at Arcmap 10 restrict Flow Accumulation, that error is apparent as well as the adjacent 35 ...

7

One approach that our amazing intern recently used, which turned out really well for us: Fill the DEM Calculate flow direction on the filled raster Accumulate the flow of the flow direction output Do a Con operation to set all cells with an accumulated flow over some threshold (that is meaningful for your geographic area and for your cell size) to be 1 and ...

7

These are common problems, typically resolved by stream burning, but that relies on you having a vector network to enforce drainage. I recommend you check out the free GIS software Whitebox GAT it has several alternative methods to fill and compute flow accumulation.

6

The method that I've implemented in a couple of languages and believe that ESRI uses (sorry, no references other than Jenson and Domingue cited elsewhere in this page) is to start at a user-supplied "pour-point" cell or a cell at the edge of the flow direction grid (fdr), examine its eight neighbors to find which of those direct flow into the current cell, ...

6

You should also take a look at TauDEM (http://hydrology.usu.edu/taudem/taudem5/index.html) It's a set of command-line .exe files (you run them in the Windows Command Prompt) for calculating flow direction, flow accumulation, and stream thresholds. The input and output of each command is a GeoTiff raster file. TauDEM can be installed stand-alone without ...

6

As someone who did feature capture from imagery for a while, I would caution you against expecting a pool at a spring. The majority of the ones I've encountered (both in capture and on the ground in person) don't have one. We often referred to ancillary sources to suggest/confirm a spring. Depending on your purposes, USGS quad sheets or hydrography datasets ...

6

You can download hydrology shapefiles from here for a number of countries around the world. Select your country of choice and Inland water as your subject. You should receive 2 types of shapefiles: A line shapefile depicting line features for rivers, canals and basins A polygon shapefile depicting area features for rivers, canals and basins I believe ...

6

In the world of hydrology and geomorphology, there is indeed a metric that we use to classify/quantify the "curviness" of a river......sinuosity. Sinuosity is simply a measure of the actual path length of the river divided by the shortest path length (straight line distance). So, you could measure the sinuosity of the river as a whole (actual path length ...

5

I made a port of part of this fortran code: function hrap(lon,lat){ var deg2rad = function (deg) { return deg * (Math.PI / 180); }; var stlat = 60.0; var clon = 15.0; var rad = 6371.2; with (Math){ var sfactor = (1+sin(deg2rad(stlat)))/(1+sin(deg2rad(lat))); var R = rad*cos(deg2rad(lat))*sfactor; var x = R*cos(deg2rad(lon+clon)); var y = R*sin(deg2rad(lon+...

5

You can approximate this using standard Spatial Analyst tools. (To do better, you will need to write the code from scratch in Python.) (The approximation is that the flow accumulation can be limited to a fixed number of steps, where each step is a horizontal, vertical, or diagonal flow across a single cell. For a cellsize of c and n steps, then, you are ...

5

Yes, I would use ArcHydro. I found a document on this page (updated link thanks to @Polygeo) a while ago, that outlines some possible workflows (from the ArcHydro Team). There have been some changes compared to ArcMap 10.0 so for me it was quite useful.

5

I think that the original poster may want to calculate for each cell the height above the first stream cell that would be reached by water flowing from the cell. So the 'nearest stream' is calculated along the downslope flow path, not euclidian distance. The references for this Height Above Nearest Drainage (HAND) are: Rennó, C. D., Nobre, A. D., Cuartas,...

5

If you run an intersection between the 2 different line feature classes, and specify the output type to be Point, then you should be able to do a count of the ids in the point table of the input lines.

5

I can think of a few ways to do this: Symbolize the lines so they have direction indicated, and manually Flip those in the wrong direction. Use an attribute if available or a calculation (such as end z > start z) to select segments going the wrong direction and Flip (GP) them. Create a geometric network of the lines and use the Flow Direction tools, setting ...

5

There is a tool in the free and open-source (GNU GPL licensed) GIS Whitebox Geospatial Analysis Tools that can identify the flowpath from any point or collection of points specified either as a ShapeFile or as a categorical raster. The tool is called Trace Downslope Flowpaths: The tool takes a D8 flow pointer (flow directions) grid as an input, which can be ...

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