nsw_lidar
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nsw_lidar [2018/12/09 21:56] – [Hydrology (Stream Network)] bushwalking | nsw_lidar [2019/02/11 08:28] – bushwalking | ||
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- | Page for documenting NSW lidar processing | ||
====== Intro ====== | ====== Intro ====== | ||
NSW Spatial Services have undertaken a program to map all of NSW using lidar (light detecting and ranging) | NSW Spatial Services have undertaken a program to map all of NSW using lidar (light detecting and ranging) | ||
For details, see information on their [[http:// | For details, see information on their [[http:// | ||
- | Elevation data can best be accessed through the [[http:// | + | Elevation data can best be accessed through the [[http:// |
+ | |||
+ | It can then processed with a GIS such as [[https:// | ||
====== Resources ====== | ====== Resources ====== | ||
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The steps below are works in progress to determine effective (the best?) ways to extract the various items out of the DEM data for use in topographic maps. Any feedback/ | The steps below are works in progress to determine effective (the best?) ways to extract the various items out of the DEM data for use in topographic maps. Any feedback/ | ||
+ | |||
+ | ===== Merge DEMs ===== | ||
+ | |||
+ | The NSW DEM data is supplied in 2km squares. The squares need to be merged into a single DEM for further operations. | ||
+ | |||
+ | While this can be done in theory using a virtual raster, I have had poor performance with this. Any operation seems to result in screen redrawing, so moving around and zooming in and out is quite slow and painful. | ||
+ | |||
+ | Instead, I generally use the the Raster- > Miscellaneous -> Merge... function | ||
+ | |||
+ | ===== Fill Sinks ===== | ||
+ | |||
+ | From the initial DEM, first step is to Fill Sinks. Otherwise you will get sinks in the middle of watercourses, | ||
+ | |||
+ | There are various related tools in the Processing Toolbox that will do this, including: | ||
+ | * SAGA : Terrain Analysis - Hydrology : Fill Sinks | ||
+ | * SAGA : Terrain Analysis - Hydrology : Fill Sinks (Wang and Liu) | ||
+ | * SAGA : Terrain Analysis - Hydrology : Fill Sinks XXL (Wang and Liu) | ||
+ | |||
+ | The results from all will be similar, but the Wang and Liu versions should be faster. | ||
+ | |||
+ | There are other approaches that deepen channels rather than fill sinks in order to get a hydrologically sound drainage network. For example | ||
+ | * SAGA : Terrain Analysis - Hydrology : Sink Removal | ||
+ | has an option for this. | ||
===== Contours ===== | ===== Contours ===== | ||
- | See page on [[[qgis_contour_labelling|QGIS Contour Labelling]] | + | ==== Basic Processing ==== |
+ | There are various contour extraction algorithms in QGIS, for example: | ||
+ | * GDAL : Raster Extraction : Contour (same as Raster -> Extraction -> Contour...) | ||
+ | |||
+ | Below is an example of contours created without and with sink removal. The contours on the right have been derived from a DEM where the sinks (in yellow on the left) have been filled. | ||
+ | {{: | ||
+ | {{: | ||
+ | |||
+ | Even with sink removal, small | ||
+ | |||
+ | ==== Simplifying ==== | ||
+ | |||
+ | Vectors can be compressed by using something like: | ||
+ | * Vector geometry : Simplify | ||
+ | A tolerance of 1(m) seems reasonable for 1:25000 mapping. Smaller tolerances may be appropriate for larger scale maps (eg 1:10000, 1:5000). | ||
+ | |||
+ | For more options in compression, | ||
+ | * GRASS : [[https:// | ||
+ | V.generalize can also be used to smooth contours - possibly best done prior to simplificiation | ||
+ | |||
+ | ==== Cleaning ==== | ||
+ | |||
+ | Once simplified, it is worth removing small closed loops, such as those in the image below. | ||
+ | {{: | ||
+ | |||
+ | Here is one approach, which involves adding a length attribute to each contour, and removing those that fall below a certain length. It may cause issues if you have short sections of contour near the edge of the map that you need. | ||
+ | |||
+ | * Open Attribute Table (F6) | ||
+ | * Open field calculator (Ctrl+I) | ||
+ | * Add new attribute length, calculated as $length | ||
+ | {{:: | ||
+ | * Select all features and filter on length < 25 (or whatever length is appropriate for your scale) | ||
+ | {{: | ||
+ | |||
+ | ==== Contour Labelling ==== | ||
+ | |||
+ | See separate | ||
===== Hydrology (Stream Network) ===== | ===== Hydrology (Stream Network) ===== | ||
- | Fill Sinks | + | The starting point for hydrology is a hydrologically sound DEM, as above. Use a fill sinks or channel deepening algorithm. |
- | From the initial DEM, first step is to Fill Sinks. There are various related tools that will do this, including: | + | ==== Catchment Areas ==== |
- | * Fill Sinks | + | |
- | * Fill Sinks (Wang and Liu) | + | |
- | * Fill Sinks XXL (Wang and Liu) | + | |
- | Catchment Areas | + | Next step is to create |
- | Next is to create Catchment Areas. Again, there is a Catchment Area tool (in fact several), and six options within the tool. For the purpose of delineating watercourses | + | * SAGA : Terrain Analysis - Hydrology : Catchment Area |
+ | |||
+ | This gives an output that is best viewed in log scale. You can do this via | ||
+ | * Raster -> Raster Calculator... | ||
+ | * log10 ( " | ||
+ | |||
+ | Use the log scale version | ||
+ | |||
+ | Note that if you don't have the entirety of the catchment, you may get erroneous results. | ||
+ | |||
+ | ==== Channel Network ==== | ||
+ | |||
+ | The following tool can be used to create channels (streams) - there are other options: | ||
+ | * SAGA : Terrain Analysis - Channels : Channel Network | ||
+ | |||
+ | Use | ||
+ | * Elevation = Filled DEM | ||
+ | * Initiation Grid = Catchment Area | ||
+ | * Initiation Type = Greater Than | ||
+ | * Initiation Threshold = 10000 (or whatever number you have determined) | ||
+ | |||
+ | {{: | ||
+ | |||
+ | ==== Classification ==== | ||
+ | |||
+ | For 1:25000 maps, I've had reasonable results from using the following formula | ||
+ | |||
+ | '' | ||
+ | |||
+ | * Intermittent: | ||
+ | * Minor: 6.15-7.4 | ||
+ | * Major: 7.4+ | ||
+ | |||
+ | ==== Convert to Vector and Simplify ==== | ||
+ | |||
+ | Convert to vector using r.to.vect | ||
+ | |||
+ | {{: | ||
+ | |||
+ | The raw stream data is very jagged. Smooth using | ||
+ | * v.generalize | ||
+ | * Algorithm = Hermite (there are other options which can be used) | ||
+ | * Maximal tolerance value = 20 (in m, obviously scale dependent) | ||
+ | |||
+ | Simplify using using: | ||
+ | * Vector geometry : Simplify | ||
+ | Tolerance:? | ||
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The steps below have been tested in the Blue Mountains, a region that has a significant number of relatively vertical sandstone cliffs. It may be less effective in different terrain. | The steps below have been tested in the Blue Mountains, a region that has a significant number of relatively vertical sandstone cliffs. It may be less effective in different terrain. | ||
+ | |||
+ | This is more a set of ideas than a fully fledged process. The main aims are to get a set of steps that can largely be automated, and that create cliffline vectors that are running in the correct direction. There is still some way to go on this! | ||
==== Initial analysis of slope, aspect ==== | ==== Initial analysis of slope, aspect ==== |
nsw_lidar.txt · Last modified: 2024/04/05 19:00 by bushwalking