# Tag Info

30

In my experience, DEM is most of the time used as a generic term for DSMs and DTMs. I think this image on Wikipedia depicts the differences between DSMs and DTMs well: DSM = (earth) surface including objects on it DTM = (earth) surface without any objects A different definition is found in [Li et al., DIGITAL TERRAIN MODELING - Principles and ...

18

Digital elevation models (DEM) are a superset of both digital terrain models (DTM) and digital surface models (DSM). Remote sensing generally captures the surface height, so the top of the tree canopy or buildings is returned, not the bare ground elevation. If this data is corrected to remove elements which extrude above the terrain height, you're left with ...

15

In my experience, a more complete answer to this question lies in defining the difference between a DEM, DTM and a DSM. A DTM is NOT a generic name covering both DEMs and DSMs. So... A DEM is a 'bare earth' elevation model, unmodified from its original data source (such as lidar, ifsar, or an autocorrelated photogrammetric surface) which is supposedly free ...

13

Curvature is a complex terrain derivative to compute, the equation that you use depends on the resolution of your input data, as you have to ensure that the curvature results you compute can be distinguished from noise in the data. A lot of research has been done recently on curvature calculations on high resolution LiDAR data which showed that a scaling ...

10

ESRI's version of Raster Analysis for calculating curvature might be helpful to develop a plugin for QGIS. For each cell, a fourth-order polynomial of the form: Z = Ax²y² + Bx²y + Cxy² + Dx² + Ey² + Fxy + Gx + Hy + I is fit to a surface composed of a 3x3 window. The coefficients a, b, c, and so on, are calculated from this surface. The ...

6

An elevation profile computes the intersection of two surfaces. One of them is a vertical sheet determined by a path. (That is, it consists of all coordinates (x,y,z) where (x,y) is on the path and z is any number.) The other is the surface represented by the raster DEM. As such, it amounts to finding the z-values lying above points on the curve. This ...

6

I would say this depends on your knowledge of the data and how it was collected as well as how the DTM was generated from the raw data (regular grid vs interpolated from irregular points or other sources) I don't think there is a rule of thumb. Personally, I choose contour intervals based on the scale, type, content, purpose of the map, etc.

5

OS Terrain 50 contours (10m contours) for Great Britain (England, Scotland and Wales) It is supplied both as a set of 50m gridded digital terrain model (OS Terrain 50 grid) and 10m contours and spot heights (OS Terrain 50 contours). Notice: OS Terrain 50 contours and OS Terrain 50 grid are now available as of 8th July 2013. Tip: Opt for the OS ...

5

digital surface model (dsm) – a first-reflective-surface model that contains elevations of natural terrain features in addition to vegetation and cultural features such as buildings and roads. digital terrain model (dtm) – a bare-earth model that contains elevations of natural terrain features such as barren ridge tops and river valleys. ...

4

If you want create a profile of maximum quality, then your algorithm has to basically include every single cell that is intersected by your query path and then it becomes a simple 2D curve fitting problem. However, if you want to just sample a subset of those points and create a profile that is more visually pleasing, you may that find that this paper from ...

4

The Ordnance Survey have Landform Profile and Landform Panorama that were based on their contour data, but it is not fantastic quality, and it is based on very old data. But it is now free to download, so if you just need a dataset to play with, it's a good choice. I've used GetMapping's NEXTMap Britain 2 lidar data, and it is of generally very good ...

4

A nice, fun problem. I'm not sure if you have access to FME or to the Data Interoperability Extension, but if so I felt inspired to create an FME solution, and you can find details about it on my FME Evangelist blog. If not, maybe others will find this (or the methodology) useful.

3

First convert your line layer into a point layer. You could do this with either the Feature Vertices To Point tool in ArcGIS or Polyline to Point tool using ET GeoWizard. Once you have a point layer in QGIS you can use the Point sampling tool to extract the elevation data from a raster. Below is a tutorial showing you how to use the Point sampling tool in ...

3

You could perform a watershed analysis yourself, but its a time consuming process. One good source of existing river catchment data is the HydroSHEDS project, which provides high resolution basins for much of the globe, including the UK.

3

The trick is to Google "UK river catchment map". (Other combinations that include "watershed" don't work.) You will find River basin management plan documents. River basin district maps (downloadable files). An interactive map. Maps by the UK Environment Agency. Scottish river basin maps and data. A search involving "watershed" did turn up a historical ...

3

If you are in the UK there is the Ordnance Survey Land-Form PANORAMA dataset that is available as part of OS OpenData. It is available as a DTM or as contours. The DTM is designed for use at a scale of 1:50,000. For more information, use the link below: http://www.ordnancesurvey.co.uk/oswebsite/products/landformpanorama/

3

This problem has many different solutions: it needs further restrictions. After all, by choosing any value within the range of elevations in the dataset, the contour for that value will be "flat" (and horizontal). This probably is not the sought-for answer, but it is an answer and it's perfectly valid. Moreover, among all possible answers, the contour ...

3

OK, take 2... If you're looking for a region of specific size and area: As before, classify your cells by the slope. Then do a Region Group on the raster. The count value will give you area values for the raster. Then if you need regions of a certain width (ie, not something long and thin) run Zonal Geometry with the Thickness option, which will give you ...

3

Your best bet would be to first calculate slope, as om_henners has mentioned, then use a focal operator to collapse the result down to the scale you're interested in. I'd recommend using FocalMean, where the neighborhood size matches the scale of 'flat' areas you're interested in. So for example, if you had a raster of 10m resolution, a 5x5 neighborhood ...

2

The minnimum contour interval is the dubble vertical error (RMSE or standart derivation) of the highmodell. You can find that for ASTER GDEM in: Lang, R. Harold, and Roy Welch. 1999. “Algotithm theoretical basis document for ASTER digital elevation models.” Its defined by the "United States National Map Accuracy Standards (NMAS)" For example: If your DHM ...

2

Do you mean 3 arc-seconds? That indicates LatLong. You will have to create a GRASS location with UTM or Lambert LAEA/LCC or another metric coordinate system and run r.inund.fluv therein. See also "GRASS GIS projection intro".

2

If you don't find a map, SAGA GIS has a module called watershed basins that uses a dem and the channel network (your rivers). If you use a coarse dem this goes fast. http://www.saga-gis.org/saga_modules_doc/ta_channels/index.html

2

Recently, I came across with this discussion and I would like to register an answer too, more specifically about LiDAR. It is common to see in Forestry LiDAR scientific articles acronyms DTM and DEM to be used as thesaurus. According to Behrendt (2012), paragraph 6 on page 15, there is a difference between them: DTM: bare-earth representation with ...

2

You were right about the spatial analyst extension. Depending on some assumptions (see below) with any luck there shouldn't be too much analysis. Take your shapefile of built areas and turn it into a grid - make sure that it's the same origin and cell size as the original DTM. Run a Combine between your built areas raster and your DTM - note that the built ...

2

First of all correlation is the degree to which two or more attributes or measurements on the same group of elements show a tendency to vary together. I think what you're looking for is the RMSE between your datasets. From wikipedia: The root-mean-square deviation (RMSD) or root-mean-square error (RMSE) is a frequently used measure of the differences ...

1

The curvature could be calculated using SAGA's module 'Terrain analysis - Morphometry ---> Slope, Aspect, Curvature' The calculation could be done based on one of these algorithms: Maximum Slope (Travis et al. 1975) Maximum Triangle Slope (Tarboton 1997) Least Squares Fitted Plane (Horn 1981, Costa-Cabral & Burgess 1996) Fit 2.Degree Polynom (Bauer, ...

1

you can check out More Color Ramps for QGIS with Colorbrewer, here from user @underdark. you can add your color code following xml file for qgis style manager.. If you want to use them, just import the following XML file into QGIS Style Manager: https://github.com/anitagraser/QGIS-resources/tree/master/symbols i hope it helps you...

1

I think this is correct and please can someone step in if it is not. I think the easiest solution is to calculate total ascent is to use excel, method below: [1] Convert the line to points [2] Export the data to a format compatible with excel [3] Make sure the track is going from start to finish - if not then reorder the data [4] Enter the formuala ...

1

If you want a comprehensive source for DTM-DSM and DEM please read this book "Digital Terrain Modeling: Principles and Methodology" Author: Zhilin Li,Qing Zhu,Chris Gold Publisher: CRC Press ISBN: 0415324629 ISBN13: 9780415324625 Pages: 323 Language: English Release Date: 2004-11-29 00:00:00

1

If you have access to Spatial Analyst you could use Slope instead of cut/fill. Calculate the slope raster Calculate your tolerance based on the maximum rate of change (see the formula on How Slope Works page) Do a setnull so that it sets the raster null where the slope values are greater than the tolerance; eg setnull(slope > tolerance, original_values)

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