OpenStreetMap

Mapping surface features with high resolution DEM generated from Lidar

Posted by abejorro34 on 1 December 2017 in English (English)

Example of use at mountain relief modified by mining and agriculture

Lidar (light radar) is a technique providing a very precise surface scanning. Airborne laser scanning generates a dense point cloud (several points per square metre) representing reflections of light ray. It contains reflections as from the vegetation as from the surface under it. Vegetation and building reflections may be filtered to get relief data only and create a digital elevation model (DEM) of relief. It is usually a raster and each pixel value represents the relief elevation. This raster may be further processed for better visualisation of relief shapes:

  1. Hillshade raster – grayscale 3D visualisation,
  2. Slope raster – colour represents the rate of change of elevation,
  3. Aspect raster – colour represents the downslope direction.

Surface irregularities cause by human activities are clearly recognizable in the high-resolution DEM obtained from Lidar.

DEM Spoil

My hometown was found in the Middle Ages after discovering a rich silver ore. Old mines were reused after second world war to get uranium. The massive mining activities caused significant changes of relief. Some valleys were filled with mining deposits. Agriculture in mountains was also present. Fields at slopes were divided with little retaining walls and embankments.

Embankments

Mapping these features may be useful for more purposes. Scientists and some tourists going away from tracks want to avoid demanding walking over mining deposits, which usually consist of rock blocks and are often overgrown with trees and shrubs. Geologists and mineral collectors would appreciate the presence of these features in map to locate old mines and spoil heaps. These mines are an important part of local history and the map would tell more about them to visitors and historians. Tourists get better image about the extent of spoil heaps. Some line relief features like embankments are good for orientation. Moreover, map with such features looks much more attractive.

Spoil heaps covered with vegetation are difficultly mappable from aerial images. Mapping with GPS in mountain terrain would be very demanding. GPS signal under steep slopes and vegetation is less accurate. Spoil heaps in mountain relief are characterized by change of the downslope direction and are well recognizable in the slope raster as in the hillshade raster. Spoil heaps are mapped with polygon and tagged as man_made=spoil_heap or landuse=landfill (I'm not sure which one is better).

Embankments, retaining walls, cliffs and edges of spoil heap terraces are characterized by abrupt change of surface inclination and thus they are well recognizable in the slope raster. Sometimes it is not clear from the slope raster, what is the downslope direction and which one is the upper edge of the slope and the mapping may be confused. I recommend checking all created features with the hillshade or aspect raster.

Line representing embankments (man_made=embankment), retaining walls (barrier=retaining _wall) and cliffs (natural=cliff) should be placed to the upper edge of the slope and the right side of the line should always face downslope. When mapping these features from DEM, you should always know which one is present. In case of old agricultural retaining walls in my hometown, most of them are ruined and can be marked as embankments. Edges between spoil heap terraces and downslopes are marked as embankments as well. Roads and tracks at slopes are mostly placed over embankments. Slope raster is very suitable for mapping roads and tracks as well, especially if the track is covered by vegetation, not visible in the aerial map and the GPS signal was not accurate.

Mapping

I checked my mapped features in field and must say, that this method is very reliable and accurate, because I made no mistakes. However, I know the area very well and was always sure which features I mapped.

I suppose that getting DEMs from Lidar in most countries is not so easy because it is not available for free. Sometimes it's available as WMS and JOSM allows to add a WMS layer. Always make sure that the provider license is compatible with OSM and you have permission to load the derived data to OSM.

Comment from imagico on 1 December 2017 at 10:48

Are you sure it is allowed to use this for mapping in OSM? Neither the WMS nor the info on the data set on

http://geoportal.cuzk.cz/(S(lhmhzafvgs5vuq3qgjkbphev))/Default.aspx?lng=EN&mode=TextMeta&side=vyskopis&metadataID=CZ-CUZK-DMR5G-V&head_tab=sekce-02-gp&menu=302

indicate this is available under a free license compatible to OSM.

Comment from abejorro34 on 1 December 2017 at 11:42

Hello, I just read this document http://geoportal.cuzk.cz/Dokumenty/Vyhlaska.pdf about terms of use. It says that the "viewing service" WMS is for free and you should just mention the source. There are some restrictions only if you would like to share the downloaded raster but there is no restriction for your own vectorized data. Other WMS layers from ČÚZK are officially used by OSM mappers and mentioned in the list of sources here: http://wiki.openstreetmap.org/wiki/Cs:Key:source#N.C4.9Bkter.C3.A9_hodnoty_.C4.8Dasto_pou.C5.BE.C3.ADvan.C3.A9_v_.C4.8Cesku I ask about it other Czech mappers and directly the ČÚZK office and will let you know.

Comment from imagico on 1 December 2017 at 11:54

Checking with other Czech mappers and the data provider is a good idea - free-to-use services do not generally mean they are free for data extraction like tracing in OSM.

Otherwise nice example of the usefulness of relief data for mapping.

Login to leave a comment