March blog visualisation – Contours and Digital Elevation Maps
As promised here is another blog on data visualisation. The previous blog looked at combining travel time contours with average speed links to learn more about congestion patterns, you can see it here.
This month the focus is on Digital Elevation Models (DEM’s) and how these can work with travel time data. What are DEM’s you ask? Well, as the name states, such models help us to understand the landscape around us using satellite imagery. A tool that is predominantly used in studies looking at erosion, geomorphology, and hydrology.
However, elevation is also a key player in journey times and how we might plan journeys. Especially for any unlucky souls (like myself) whose car doesn’t have a whopping 986 horsepower and can barely make it over the hill! This is also important for electric vehicles, something our EVR product considers. But let’s not forget how elevation might be a particular challenge for cyclists. Cycling is not only a hobby, but essential travel for many people so often we have to consider how speed can be affected by elevation and as a result can impact journey time.
For this month’s map we combined a travel time contour with a 3D DEM. Firstly, we modified the cycling speeds in the road network as these only come with default speeds. These were changed in TRACC to more appropriate speeds based on elevation.
From this a contour was produced showing the travel times for cycling to Coldharbour in Surrey as shown in image one. The contour was then placed on top of a 3D DEM, which was created in an alternative GIS software. The great thing about using DEM’s is that they are easily customisable to every project. They also provide vast geographic coverage, making them extremely useful for much larger projects.
To improve the travel time output further, the road network was also combined with the DEM. This makes it easier to visualise where road links sit in relation to areas of high elevation. For this, another map was created with a customised road network displaying the varying speeds based on elevation.
By doing this we were able to get a complete visualisation of correlations between journey times and elevation.
We’ll be back next month for another data visualisation.
A bit of extra reading…
The SurfZone Digital Elevation Model (DEM), produced in 2014 to model the coastal zone. Using the best available LIDAR and near-shore multibeam SONAR bathymetry available at the time, it is the best currently available Digital Elevation Model (DEM) covering the inter-tidal zone produced by the Environment Agency.
Produced by using a bespoke feathering technique to smooth the overlaps between LIDAR and Bathymetric surveys to produce a seamless surface. Small gaps between surveys were interpolated to produce a full coverage DEM of the near-shore. For all regions except Anglian Digital Terrain Models (DTM) derived from LIDAR were used. For the Anglian coastline a Digital Surface Models (DSM) were used.
What are DEMs?
Digital elevation models (DEMs) are arrays of regularly spaced elevation values referenced horizontally either to a Universal Transverse Mercator (UTM) projection or to a geographic coordinate system.
Today, DEMs are usually generated from remotely sensed data sets collected either from an aircraft (airplane, helicopter or unmanned aircraft / drone) or spacecraft (satellite or Space Shuttle). One of three types of digital sensors typically collect data that can be used to derive elevation measurements – optical imaging sensor (frame camera / push broom scanner), synthetic aperture radar (SAR) sensor, or laser scanner (also called LIDAR) sensors. These technologies are dramatically different from one another.
DEMS are a valuable tool for:
- Modelling water flow for hydrology or mass movement
- Extracting terrain parameters for geomorphology (the study of physical features on earth’s surface)
- Erosion and drainage analyses
- To set viewpoints for fire detection.
- Construction planning – Evaluate areas, create a visualisation, and estimate costs.
- Further info about this on here https://www.digitalglobe.com/products/advanced-elevation-suite might be useful.