Press 'o' to toggle the slide overview and 'f' for full-screen mode.

Copyright © John Lindsay, 2015

Methods based on parallax

• The effect whereby the position or direction of an object appears to differ when viewed from different positions.
• These are the most useful approaches to measuring heights from images.
• Requires high quality stereoscopic photo pairs (two overlapping images).
• Image tilt must be less than 3 degrees.
• Images must be taken from the same height.

Parallax

Parallax

Steroscopic viewing methods

Lens Stereoscope with Parallax Bar

Parallax equation for level terrain

• h = height of the object being measured
• H = flying hgt. above the object base
• P = Average absolute parallax (avg. Distance between PP and CPP)
• dP = difference in parallax between top and bottom of the object

Parallax equation for mountainous terrain

• ΔE = Elevation difference between the object base and the average of the two PPs, + if higher and – if lower.

Measuring P

Measuring dP

Measuring dP

Using a parallax bar to
measure differential parallax

Extraction of Building Infrastructure Using
Soft-Copy Photogrammetric Techniques

Digital Elevation Models (DEMs)

• You can calculate the parallax for any location on stereo-images.


• It is commonly the case that this would be done along a regular grid and these x, y, z points then used to create DEMs.

Digital Elevation Models (DEMs)

Digital Elevation Models (DEMs)

• In the field of Computer Vision (stereo-vision), a disparity map/image is equivalent to a DEM...measures the distance from the camera.


• Often calculated using OpenCV or Matlab software and from images taken by uncalibrated cameras.


• These frequently lack the precision of a survey-grade DEM.

Disparity Map

Photogrammetry isn't just
about measuring heights

• It also includes measuring heights, angles, distances, and areas.


• Why might you want to measure an area off imagery?

The challenges...

• It's easy to measure the area of simple shapes but what about more complex shapes?


• Images are not maps and are affected by distortions and displacements.


• Raw images have non-uniform scale throughout.

Areas of simple shapes

But what about the area of this?

Methods for measuring areas

• Weight apportionment method
  • Calculates the percent coverage of a particular land cover compared to total
  • Need a very precise balance


• Dot count method
  • Transparent overlay with fine grid of dots

Methods for measuring areas

• Planimeter
  • Specially designed instrument to measure perimeters and areas
  • Must convert to ground units.


• Digitizing tablets


• On-screen digitizing

On-screen digitizing

• Either scan a hardcopy image or use digital imagery.


• Digitizing takes place in a geographic information system (GIS).


• Each digitized polygon becomes a record in a GIS attribute table.


• The attribute (e.g. Forest, farmland), perimeter, and area are easily queried from the table.

On-screen digitizing

• Best to geometrically rectify before you digitize!


• This is the process of creating an orthophoto to serve as the base for digitizing.

Orthophotos/Orthoimages

• Convert images with perspective projections (conical bundle of rays) to an orthographic projection (parallel rays)

Orthophotos/Orthoimages

• The effects of tilt and relief are removed.


• An orthophoto is a uniform-scale photograph. It is a photographic map.


• Since an orthophoto has a uniform scale, it is possible to measure directly on it like other maps.

Orthophotos/Orthoimages

• The accuracy of a digital orthoimage is a function of:
  • The quality of the raw imagery
  • The ground control points (GCPs)
  • The quality and character of the DEM

Orthophotos/Orthoimages

• Orthorectification is challenging in areas of rapid elevation change, which is common in urban areas containing tall buildings.


• So called 'True' orthoimages not only remove topographic displacement due to the terrain, but also the effects of off-terrain objects.


• This requires a digital surface model (DSM) rather than a bare-Earth DEM.