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## GEOG*2420

John Lindsay
Fall 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

(From: Jensen 2007)

### Steroscopic viewing methods

(From: Jensen 2007)

### Lens Stereoscope with Parallax Bar

(From: Jensen 2007)
(From: Jensen 2007)
(From: Jensen 2007)

Notice that the average absolute parallax (Distance between PP and CPP) is the image distance equivalent to the air base.

### Parallax equation for level terrain

$$h = \frac {H{dP}}{P + {dP}}$$

• 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

(From: Mersey)

### Parallax equation for mountainous terrain

$$h = \frac {H{dP}}{P + \frac {P \Delta E}{H} {dP}}$$

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

(From: Mersey)

(From: Mersey)

### Measuring dP

(From: Mersey)

(From: Jensen 2007)

(From: Jensen 2007)

(From: Mersey)

### Extraction of Building Infrastructure Using Soft-Copy Photogrammetric Techniques

(From: Jensen 2007)

(From: Jensen 2007)

### 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)

• 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.

(From: Sepehri)

### 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.

### 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.

(From: USGS)

### 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.
True Orthophotos/Orthoimages
(From: Nielsen, 2004)