Digital Elevation Models in GIS (theory) - updated

hello my name is hunts from the crust lecturer at ihe Delft Institute for water education in this lecture I'm going to show you what digital elevation models are that we use in GIS after this lecture you'll be able to define what is a DM a DTM or a dsm to describe different methods of DM acquisition to give examples how DMS can be used to describe what data can be derived from DMS digital elevation models can be defined as DTM's digital terrain models where we only consider the terrain surface this is often using catchment hydrology where we want to route water over the terrain the catchment the digital surface model however is the DTM plus all natural and human-made features on top of it such as trees and buildings as we can see on the picture this is often useful for hydraulic modeling here we see a digital surface model of the city center of Rotterdam where we use a 50 centimeter elevation model and aerial photographs draped over it you can also generate a digital service model based on vector data in that case the building shapes are much clearer than when we use raster but we don't have the information of the texture of the buildings this is data from Rotterdam based on city GML data that can freely be downloaded from the internet so how do we acquire DMS now the traditional way is using ground surveying where we extrapolate from a known Xyz point to other locations where we accurately measure the position using a theodolite this requires very skilled people and it's also very laborious to cover a large area especially in mountains as you can see on the picture and in the end all these points need to be interpolated if we want to use it as a continuous raster in for example hydrological modeling more digital way is using a differential GPS there we use satellite information to determine our position very accurately in combination with a base station and in this way we can get centimeter accuracy but still points need to be surveyed and it's a lot of work to interpolate these points to a continuous raster to be used in modeling another traditional ways to use stereophotogrammetry on the picture you see a stereoscope and you see a stereo pair of aerial photographs they have an overlap in the overlapping area we can see through this device in 3d and there are special rulers that we can use to measure the elevation of points we see on the on the photographs of course that's also very laborious and you need good skills to do that and then you in some way need to digitize those points to get a restaurant of it also very laborious is digitizing contour line so many maps have contour lines printed on it and we can use a device to digitize these contour lines a lot of manual work and then we can interpolate those contour lines to a rest our that we can use in the models more modern way is to use lidar or laser altimetry with this method the laser scanner is mounted to an aircraft and it sends pulses to the earth which are reflected back and the reflection the time difference of the reflection is recorded and from that together with GPS information and the rotation of the plane we can derive the elevation of the surface this generates a lot of very accurate points which need to be post processed and also interpolated to get a raster if we don't have access to all these other acquisition methods we can always use the radar interferometry method there has been the shuttle radar topography mission I'll talk more about it later which acquired a lot of data almost full coverage of the the earth at a an acceptable resolution for catchment modeling for example so how do we use DMS we can use DMS to determine the catchment area and to delineate drainage networks which is covered in a separate video we can calculate slopes we can calculate the aspect which is the orientation of slopes according to the compass direction that's very useful for applications where we want to know the amount of solar energy that is received by a slope so North slopes for the South slopes for example we can use DMS to identify geological structures because when there are abrupt changes in the elevation it's an indication of a change in your geology if used for view shet analysis few shots are the areas that can be seen from a point or to determine which point you can see from a certain area very useful for military purposes or for spatial planning another use of DMS is or to rectification ortho rectification is the geo referencing of aerial photographs or satellite images where we take into account the role EF displacement that is an effect of this kind of images you can also use obviously DMS for 3d simulations like landslides mass movements you can use it for change analysis and we can use it to create contour maps I will illustrate the use of DMS using an example from the French Alps this is an area near de LaBelle where our students from ihe Delft specialization hydrology do their fieldwork during this fieldwork the students have to study the hydrological processes in their own study areas their catchments prior to going to the fieldwork it can be useful to study first the digital elevation models of those catchments in order to understand the relation between hydrology and elevation differences it can help them in understanding that are their study area and to prepare their sampling strategy while they are in the field also when coming back from the field the digital elevation model can help them in further interpretation of the study area here we look at the digital elevation model in the form of a roster each cell represents the elevation value and we use a color ramp to give two at different elevations different colors note that this is a continuous raster without sharp boundaries and therefore ramps are used also make sure that you use intuitive colors so blue is normally associated with low areas as well as green well the darker colors or even white for snow are associated with higher areas we can also calculate the hill shade in a GIS in this case an artificial light source illuminates the scene normally the light source is put in the northwest which doesn't exist in reality however if we put it in the southeast for example this will give an inverted really F as you can see in the animation a nice visualization trick is to combine this raster DM with a hill shade and in that case we get this more dramatic view we can either use transparency but more impressive is it to use the blending if your software supports this this is making huges elevation data can also be visualized using contour lines contour lines can also be derived from DM rosters the contour lines are formed by connecting locations with the same elevation is done with a certain equidistance the elevation difference between two lines can you see what is used in this case here an equidistance of 50 meter is used what we can also see in a contour line map is when the lines are close together that the area is very steep and when they widen further apart that it gets less deep so we can interpret also the shapes of the landscape from contour line map we can also visualize elevation in so-called two and a half D it's not real 3d because then we need special devices but the effect of 3d is created by perspective and by shading energy is we can also visualize this in animations which gives us further insight in the study area besides looking at the color of a DM we can also drape an orthophoto or a satellite image of the DM like in this case and here we see the animation of the area around the yellow bear where we can further interpret the shape of the hills the geomorphology for hydrological applications in a GIS we can also calculate the slope from DMS we can choose between slope in degrees or in percentage here we have calculated it in degrees and we have blended the result with the hill shade to better interpret the results and the color scale from blue to red the more red it is the steeper the slope another layer that we can derive from digital elevation models is the aspect aspect is the orientation of the slope and usually reflected in compass direction note that in this case we need a circular or directional legend where north and south and east and west are opposite the aspect is a useful map because the orientation of the slope determines the amount of solar radiation that is received by the slope and that affects the hydrology and growth vegetation for example which also affects erosion and weathering in this example we see a view shet you have calculated the areas that are visible from the center of Daniela man these are indicated in red you can also derive more complex things from DMS by applying equation one example is the topographic wetness index which is a function of the upslope contributing area divided by the tangent of the slope and then the natural logarithm of it it indicates as you can see in the layer which areas are dry and which are expected to be wetter the G is also comes with a lot of analysis tools such as creating transects in this example we see a transect through an open pit mine and the red line gives the elevation before using the so called fill sinks algorithm that we use in hydrology which is indicated by the green line in another video I'll explain how to do the catchment delineation using a GIS I hope this video was useful for you and you learned a lot about using digital elevation models if you want to learn more please subscribe to my youtube channel or go to GIS opencourseware dot org to see more free materials and links to the courses that we give at ihe Delft

Channel: Hans van der Kwast

Views: 18,495

Rating: 4.9395771 out of 5

Keywords: GIS, QGIS, ArcGIS, DEM, slope, TWI, viewshed, 2.5D

Id: pnAdasqHxGk

Length: 11min 56sec (716 seconds)

Published: Mon Dec 23 2019