17: Capturing Elevation Data

Question 1

Why collect elevation data?

Answer 1

Topography is important for describing:

• landforms
• changes over time
• modelling
• underpins many GIS uses

Question 2

POINT CLOUD

Answer 2

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)

Question 3

LIGHT DETECTION & RANGING (LiDAR)

Answer 3

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

Question 4

AIRBORNE LiDAR SCANNING

Answer 4

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

Question 5

STRUCTURE FROM MOTION (SfM)

Answer 5

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)

Question 6

SATELLITE APPLICATIONS

Answer 6

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

Question 7

Airborne scanning advantages and disadvantages

Answer 7

Advantages:

• rapid data collection
• wide data coverage
• high accuracy
• multiple return signals
• long distance

Disadvantages:

• expensive
• equipment intensive
• data storage intensive

Question 8

TERRESTRIAL LiDAR SCANNING

Answer 8

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

Question 9

Airborne laser scanning advantages and disadvantages

Answer 9

Advantages:

• rapid data collection
• wide data coverage
• high accuracy
• multiple return signals
• long distance

Disadvantages:

• expensive
• equipment intensive
• data storage intensive

Question 10

Terrestrial laser scanning advantages and disadvantages

Answer 10

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

Question 11

SfM advantages and disadvantages

Answer 11

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)

Question 12

Radar advantages and disadvantages

Answer 12

Advantages:
-can detect surfaces through clouds, independent of weather and works at night

Disadvantages:
-not as high resolution

Question 13

Imaging by Radar

Answer 13

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