17: Capturing Elevation Data

Topography is important for describing: • landforms • changes over time • modelling • underpins many GIS uses

A ‘cloud’ of points in 3D space -represents 3D surface made of 100s to millions of points Simple data structure • point number • X,YZ coordinates • may also record colour (RGB) Can be converted to other formats: • polygon mesh • Triangulated area network (TIN) • Digital elevation model (DEM)

a laser pulse is emitted and the time of reflection from the earth’s surface is recorded -knowing speed of light can calculate distance lidar can capture multiple ‘returns’ -last return represents ‘bare’ earth • builds point cloud using laser altimetry • airborne or terrestrial systems commonly used in: geology, glaciology, forestry, archaeology, conservation

Requires: • laser altimeter • dGPS (to know location of laser as the plane moves) • intertial management system (IMS) (to know pitch/roll of aircraft and angle to ground) • aerial platform: plane/helicopter/UAV Typically: • vertical measurement • moving • side to side sweep • 3060 deg scan angle

point cloud creation from photos photogrammetry -terrestrial/aerial no lasers but instead • collection of overlapping photos • computer algorithm to calculate geometry • “photo cloud” • known ground control points (GCPs) to provide a datum/scale Requires: • a series of photos of the subject/area • 80% overlap • slight angle changes • software to process (Agisoft photoscan)

all techniques can also be applied from satellites LiDAR: ICESat-1, ICESat-2 (since 2018) Radar: Shuttle radar topography mission (SRTM), Tandem-X Photogrammetry: Aster, Worldview

Advantages: • rapid data collection • wide data coverage • high accuracy • multiple return signals • long distance Disadvantages: • expensive • equipment intensive • data storage intensive

Requires: • TLS set up • dGPS • Still camera • if mounted on vehicle, some form of IMS Typically: • horizontal measurement • fixed spot • max 4km distance • 360 deg. coverage

Advantages: • rapid data collection • wide data coverage • high accuracy • multiple return signals • long distance Disadvantages: • expensive • equipment intensive • data storage intensive

Advantages: • cheaper • far higher accuracy (not moving) • less equipment intensive • 360 deg x 90 deg coverage Disadvantages: • shorter distance • smaller coverage • holes/shielding in data

Advantages: • simple data collection • relatively inexpensive • combined with hi-res imagery Disadvantages: • computationally intensive • lots of post processing of data • no bare earth return (no DEM)

Advantages: -can detect surfaces through clouds, independent of weather and works at night Disadvantages: -not as high resolution

Across the image swath the angle changes because of elevation and therefore a phase variation is measured across the swath. Two satellites are needed. Example: Tandem-X satellite mission