GeoSpace International teams with Hexagon and RiskSpace to create DSMs and DTMs for all South Africa; imagery preferred to lidar for project area of 1.2 million km2
In 2022, a leading South African geospatial services firm identified a growing need for higher-resolution elevation data to satisfy applications in multiple sectors across the nation. A 25-meter digital elevation model (DEM) had been developed years before, but the mapping community in South Africa required 1- and 2-meter resolution elevation data for modern applications.
The geospatial firm GeoSpace International1 had been awarded numerous surveying and mapping projects by the country’s national mapping agency, the Chief Directorate: National Geo-Spatial Information (CD:NGI). GeoSpace saw a unique opportunity to drive a major new DEM project that would benefit end users throughout South Africa.
Fortunately, much of the high-quality aerial imagery that could serve as the basemap for the project was already collected. Annually, CD:NGI puts out to tender the capture of aerial imagery and orthophotos, covering several quarter-degree blocks. In 2017 GeoSpace began capturing imagery for CD:NGI using the Leica DMC III airborne mapping sensor. Seven years later, it had flown the entire land area of the country, 1.2 million square km2, at a resolution of 25 cm.
GeoSpace saw this imagery as the foundation for the higher-resolution DEM datasets and sought partners to make the nationwide project a reality.
Building a solutions partnership
The aerial imagery was an ideal start to the elevation project due to the DMC III’s large-format design. Made for photogrammetric mapping, it captures aerial data with high accuracy both horizontally and vertically, and achieves a GSD of 25 cm from an altitude of 20,000 feet AGL with 60/25 percent overlap. It is a faster and more cost-effective means of mapping elevation over large areas than airborne lidar techniques, which are designed for smaller projects.
Figure 1: RGB (left) and NIR (right) point cloud data sets were generated using the HxMap software. Source: Hexagon.
The desired 3D elevation data could be extracted from the imagery that had already been captured. The challenge, however, was processing the massive volume of raw imagery to derive the elevation products. The computer power and processing capabilities required for the project and its short timeline of two years would be enormous and did not exist in South Africa.
“We are an aerial mapping company and did not have the resources to handle an elevation data processing project of this scope,” said Bernard Jacobs, Director of GeoSpace. “The Hexagon Content Program, on the other hand, already had the resources in place to do it.”
Figure 2: Orthoimagery (top) overlaid with NDVI (bottom) to indicate vegetation. Source: RiskScape.
The Hexagon Content Program2 was already completing projects at this scale in North America and Europe. It had the processing software, computer infrastructure, and knowledge to generate the elevation models from the imagery for all of South Africa. The Leica HxMap post-processing workflow is a key component of these processing capabilities.
As Hexagon’s reseller in Africa, GeoSpace conferred with Hexagon to collaborate on the elevation project. The structure for the partnership already existed in the Content Program, which engages acquisition partners to collect aerial data. Meanwhile, Hexagon conducts the data processing using standardized cloud-based workflows to deliver consistently high-quality products regardless of geographic location.
Figure 3: Variation in vegetation roughness shown over grass or ground-based vegetation and trees/bushes or non-ground vegetation. Orthoimagery (left) overlaid with the rougher non-ground vegetation (center) and smoother ground-based vegetation (right). Source: RiskScape.
“Hexagon had the processing capacity available in the cloud to partner with GeoSpace on the project,” said Stephen Minnaar, MEA Sales Manager at Hexagon. “We essentially used the same standards, workflows and procedures developed for the Content Program and applied them to the South Africa DEM project.”
To assist with the extraction of vegetation layers and building footprints in the project, GeoSpace partnered with RiskScape3, a South African geospatial and actuarial company which specializes in the application of machine-learning algorithms for determining risks related to disasters, climate change, and other phenomena.
Deriving elevation from aerial imagery
As GeoSpace continued collecting aerial imagery with the DMC III, Hexagon began processing the four-band data to initiate the photogrammetric extraction of elevation points. Working with the HxMap software, Hexagon’s photogrammetrists generated a point cloud and an initial digital surface model (DSM) using the semi-global matching process.
The point cloud contained surface elevation values at a spacing of 25 cm on the same grid system that divided the nation into 439 quarter-degree square blocks of about 2800 km2 each. Because the points were determined photogrammetrically, they represented elevations of surface features, including trees, building structures and other objects on the ground, but the files also contained the spectral data (red, green, blue, NIR) from the raw imagery that would assist in mapping the bare-earth terrain.
Figure 4: Building footprints extracted and created by the machine-learning model. Source: RiskScape.
Hexagon delivered the DSM and point cloud to RiskScape for additional processing. The firm applied filtering and automated classification techniques to the point cloud to differentiate non-ground from ground features — a key aspect of generating bare earth or digital terrain models (DTM).
RiskScape’s technicians initially applied automated feature classification algorithms to the point cloud to separate out water, vegetation, and building features. A normalized difference vegetation index (NDVI) was then used to distinguish vegetation by type and also differentiate non-vegetation ground surfaces.
“Being able to extract vegetation information from the point cloud was quite critical in getting rid of the non-ground information,” said Thorgny Fjastad, GIS Specialist at RiskScape, adding that this process did not create a final DTM because not all vegetation is above ground level.
Figure 5: Orthoimagery (top) overlaid with NDVI (bottom) to indicate vegetation. Source: RiskScape.
Grass, for instance, essentially represents the ground or terrain elevation because it is short. RiskScape used a surface roughness filter, derived from the initial DSM, to further distinguish grass at ground level from bushes and trees above the ground. Grass typically has a smooth texture compared to other vegetation. This prevented short vegetation from being unnecessarily filtered from the DTM generation process.
In the final phases of generating the DTM and supporting data sets, RiskScape applied machine-learning algorithms to extract building footprints. The firm also extracted water bodies and gradients from the point cloud and then delivered the DTMs to Hexagon along with data layers containing the surface roughness categories, building footprints, and vegetation classes for the generation of product deliverables.
Figure 6: Final 1-meter DSM product. Source: Hexagon.
Hexagon merged all these feature layers with the point cloud data into a “pseudo-image,” or raw DSM, colorised with the orthoimagery’s spectral data. Hexagon’s photogrammetrists performed additional classification of the ground and non-ground points to generate the final DTMs, with 95% of all features more than 1.5 m in height removed.
“The team ultimately produced a 1-meter DSM as well as 2- and 5-meter DTMs nationwide,” said Minnaar. “The resulting products constitute the first homogeneous, countrywide and current commercial elevation dataset available for South Africa.”
Figure 7: Final 2-meter DTM product. Source: Hexagon.
The DSM and DTM datasets provide the accuracy and detail required for projects related to 5G telecommunications, defense and intelligence, hydrologic analysis, digital construction and planning, and more.
Søren Vosgerau Jespersen is Vice President Operations, EMEA, Airborne Mapping Services, within the Geosystems division at Hexagon, where he leads operational activities across the region. He brings more than 25 years of experience in surveying, mapping, and geospatial services, having held senior leadership and project roles in Denmark and internationally.