The impact of extreme weather events and subsidence on Texas communities is proving unpredictable. The urban sprawl surrounding Houston and its suburbs siphons groundwater from natural aquifers – this in turn leaves the land empty, causing it to collapse in on itself.

Hurricanes and deluges bring relentless flooding to an area that is literally sinking. Each year, the state holds the highest record of flood-related deaths and damages nationwide. Parts of Texas have been identified as the most flash-flood prone areas in the United States. When managing water-related issues, geospatial data – particularly lidar elevation data – is crucial. Texas Natural Resources Information System (TNRIS) has a geospatial strategic mapping programme (StratMap) that supports regional and local mapping efforts.

In 2017 TNRIS recognised the need for updated lidar data covering Houston and the surrounding area. With the number of flood control and subsidence issues increasing, other partners joined funding for the project, including the United States Geological Survey (USGS), Harris County Flood Control District (HCFCD) and Houston Galveston Area Council (HGAC). The project was contracted through Fugro’s Texas Department of Information Resources contract (DIR-TSO-3391).

We won a competitive bid to deliver high-resolution lidar elevation data over the 9,168 square mile project area, which includes Orange, Matagorda and Harris counties. Shortly after we were awarded the project, Hurricane Harvey swept in, leaving massive flooding and devastation in its wake. This need for elevation information became critical – the data would be used not only for flood modelling, but also for post-storm remediation, delineating hurricane-induced change and identifying new hazard zones.

When the waters receded, we mobilised our acquisition crew. Equipped with four planes, our crew succeeded in acquiring four points per square metre (ppsm) lidar data at better than 10 cm vertical accuracy RMSEz. Despite inclement weather events and the need for extensive coordination in high-traffic airspace, our crew collected lidar data for the entire area during the three-month flight season (2017-2018).

To overcome the challenges of acquiring data in the heavily urbanised area, we combined our experience on previous TNRIS projects with the most current lidar collection technologies to ensure delivery of a product that improves how TNRIS mitigates water-related issues. Typically, downtown areas with tall building heights result in reduced lidar point returns at the building footprints – however, we implemented a collection plan that mitigated this common discrepancy to ensure full coverage.

The data was processed to TNRIS specifications in order to guarantee a seamless integration with neighbouring datasets. The deliverables include a classified point cloud, hydro-flattened digital elevation model (DEM), hydro breaklines, intensity imagery and hillshades that will be publicly available via TNRIS.

Before delivery, the data underwent an independent third-party QAQC review for quality, completeness and accuracy. The accuracy of the data was confirmed to be much greater than the requested TNRIS specification. This was a result of Fugro's customised approach to positioning and processing the lidar data. We have worked extensively in the state of Texas, where we have held multiple indefinite delivery/indefinite quantity (IDIQ) contracts dating back to the mid-1990s.