17: Capturing Elevation Data Flashcards
Why collect elevation data?
Topography is important for describing:
- landforms
- changes over time
- modelling
- underpins many GIS uses
POINT CLOUD
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)
LIGHT DETECTION & RANGING (LiDAR)
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
AIRBORNE LiDAR SCANNING
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
STRUCTURE FROM MOTION (SfM)
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)
SATELLITE APPLICATIONS
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
Airborne scanning advantages and disadvantages
Advantages:
- rapid data collection
- wide data coverage
- high accuracy
- multiple return signals
- long distance
Disadvantages:
- expensive
- equipment intensive
- data storage intensive
TERRESTRIAL LiDAR SCANNING
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
Airborne laser scanning advantages and disadvantages
Advantages:
- rapid data collection
- wide data coverage
- high accuracy
- multiple return signals
- long distance
Disadvantages:
- expensive
- equipment intensive
- data storage intensive
Terrestrial laser scanning advantages and disadvantages
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
SfM advantages and disadvantages
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)
Radar advantages and disadvantages
Advantages:
-can detect surfaces through clouds, independent of weather and works at night
Disadvantages:
-not as high resolution
Imaging by Radar
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
