orthorectification and triangulation

Orthorectification and Orthorectification and TriangulationTriangulation

Orthorectification and TriangulationOrthorectification and Triangulation

Topics to discussTopics to discuss• Orthophotos Orthophotos

what are they?what are they? How are they produced?How are they produced?

• Forward and Backward ProjectionForward and Backward Projection

• True OrthoTrue Ortho


OrthophotosOrthophotos

What is Orthorectification??What is Orthorectification??- The process of removing geometric error:- The process of removing geometric error:- Camera orientation- Camera orientation- Systematic error associated with camera/lens distortion etc.- Systematic error associated with camera/lens distortion etc.- Relief displacement- Relief displacement- Earth curvature- Earth curvature


Generated by an elevation model (DEM) and an aerial photo. The elevation Generated by an elevation model (DEM) and an aerial photo. The elevation model can be generated by the same stereoscopic photographs, so no model can be generated by the same stereoscopic photographs, so no other source of data is needed.other source of data is needed.

Shows photographic detail without errors caused by tilt and relief Shows photographic detail without errors caused by tilt and relief displacement.displacement.

Orthographic projection rather than central perspective properties.Orthographic projection rather than central perspective properties.

An orthophoto combines the advantages of a map (constant scale, An orthophoto combines the advantages of a map (constant scale, orthographic projection) and those of a photo (photographic realism andorthographic projection) and those of a photo (photographic realism andlevel of detail).level of detail).


Producing and OrthophotoProducing and OrthophotoOrthophotos are produced by first obtaining or generating a DEM of the Orthophotos are produced by first obtaining or generating a DEM of the area. This elevation information is then used to remove the elevation area. This elevation information is then used to remove the elevation effects (the relief displacement) from the perspective image byeffects (the relief displacement) from the perspective image byreprojection.reprojection.

There are two basic approaches to generating orthophotos; forward There are two basic approaches to generating orthophotos; forward projection and backward projection.projection and backward projection.


Forward Projection - Forward Projection -

Pixels in the source image are projected onto the DEM and their object Pixels in the source image are projected onto the DEM and their object space coordinates (XYZ) are determined; the object space pointsspace coordinates (XYZ) are determined; the object space pointsare then projected orthographically onto the orthoimage plane are then projected orthographically onto the orthoimage plane (Z=0).(Z=0).

Since the spaces between the points projected into the orthoimage vary Since the spaces between the points projected into the orthoimage vary due to the terrain variation and perspective effects, the final due to the terrain variation and perspective effects, the final

orthoimage pixels must be determined by interpolating between the orthoimage pixels must be determined by interpolating between the projected points.projected points.


Backward Projection -Backward Projection -

The object space coordinates (X,Y) corresponding to each pixel in the finalThe object space coordinates (X,Y) corresponding to each pixel in the finalorthoimage are calculated. The elevation (Z) for each of these XY orthoimage are calculated. The elevation (Z) for each of these XY points are then obtained from the DEM, and the resulting XYZ points points are then obtained from the DEM, and the resulting XYZ points are projected into the source image to find out from which image are projected into the source image to find out from which image point (x’,y’) to obtain the grey value for the orthoimage pixel.point (x’,y’) to obtain the grey value for the orthoimage pixel.

Since the projected object space coordinates will not fall exactly at pixel Since the projected object space coordinates will not fall exactly at pixel centers in the image, interpolation must be done in the source centers in the image, interpolation must be done in the source image.image.


True Ortho -True Ortho -

Orthophotos that only rely on Orthophotos that only rely on DEM/DTM data will only take away the DEM/DTM data will only take away the relief displacement effect from the relief displacement effect from the terrain elevation, all other protruding terrain elevation, all other protruding objects (trees, buildings etc.) will still objects (trees, buildings etc.) will still show their radial displacement in the show their radial displacement in the final orthophoto.final orthophoto.

To reduce these effects too, and To reduce these effects too, and achieve a so called ”True Orthophoto”, achieve a so called ”True Orthophoto”, there is need for DSM data.there is need for DSM data.


True Ortho –True Ortho –

True orthos often have problem with blind spots and missing data.True orthos often have problem with blind spots and missing data.


Triangulation – –

In trigonometry and geometry, triangulation is the process of determining the location of a In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly. The point can then be fixed as the third point of a triangle measuring distances to the point directly. The point can then be fixed as the third point of a triangle with one known side and two known angles.with one known side and two known angles.

Triangulation can also refer to the accurate surveying of systems of very large triangles, Triangulation can also refer to the accurate surveying of systems of very large triangles, called triangulation networks. called triangulation networks.

Triangulation is a way of determining something's location using the locations of other Triangulation is a way of determining something's location using the locations of other things. It is commonly used by geologists to find the locations of Earthquakes, and is also used to things. It is commonly used by geologists to find the locations of Earthquakes, and is also used to determine spacecraft location. There are several ways to use triangulation to figure out location. determine spacecraft location. There are several ways to use triangulation to figure out location. Here's how it works. Here's how it works.

Triangulation is the principle used by photogrammetric technicians to produce 3-dimensional Triangulation is the principle used by photogrammetric technicians to produce 3-dimensional point measurements. By mathematically intersecting converging lines in space, the precise location point measurements. By mathematically intersecting converging lines in space, the precise location of the point can be determined. If the XYZ coordinates of the points on the object are known, we of the point can be determined. If the XYZ coordinates of the points on the object are known, we can compute orientation. can compute orientation.


Triangulation Example -Triangulation Example -

A set of 4 QuickBird images of the Phoenix, Arizona area, with A set of 4 QuickBird images of the Phoenix, Arizona area, with support data and ground control points were provided by support data and ground control points were provided by DigitalGlobe for purposes of validating the RemoteView DigitalGlobe for purposes of validating the RemoteView triangulation with the rigorous QuickBird sensor model. The block triangulation with the rigorous QuickBird sensor model. The block consists of 2 images from one pass and 2 images from another consists of 2 images from one pass and 2 images from another pass. Within each pass, the initial relative orientation of the pass. Within each pass, the initial relative orientation of the images is quite good, within about 10 meters. Across the paths images is quite good, within about 10 meters. Across the paths however, there is a large misalignment of nearly 700 meters. however, there is a large misalignment of nearly 700 meters. Figure 4 shows one of the cross-track stereo pairs, before Figure 4 shows one of the cross-track stereo pairs, before adjustment.adjustment.

QuickBird Stereo Pair before adjustment.QuickBird Stereo Pair before adjustment.Note the large displacement in the north-south direction as seen in the major highway. Figure 5 shows the sameNote the large displacement in the north-south direction as seen in the major highway. Figure 5 shows the samestereo pair after block adjustment. For this adjustment, no ground control points were used. 24 tiepoints werestereo pair after block adjustment. For this adjustment, no ground control points were used. 24 tiepoints weregenerated automatically in approximately 1 minute. The relatively large amount of time required to generategenerated automatically in approximately 1 minute. The relatively large amount of time required to generatetiepoints reflects the poor initial Relative Orientation of the images.tiepoints reflects the poor initial Relative Orientation of the images.

Figure 5. QuickBird stereo pair after adjustment.


Before and after block adjustment for the entire 4-image block data is shown in Before and after block adjustment for the entire 4-image block data is shown in Table Table 2. For this adjustment, 1 GCP and 32 tiepoints were used. Two GCPs were 2. For this adjustment, 1 GCP and 32 tiepoints were used. Two GCPs were

used as tiepoints to compute the RMS errors. The adjustment required 4 used as tiepoints to compute the RMS errors. The adjustment required 4 iterations and completed in under 15 seconds, computing 24 sensor iterations and completed in under 15 seconds, computing 24 sensor

parameters and the coordinates of all tiepoints and checkpoints.parameters and the coordinates of all tiepoints and checkpoints.


Sources for further study:Sources for further study:

http://web.pdx.edu/~jduh/courses/geog493f08/Students/VTeeters.pdfhttp://web.pdx.edu/~jduh/courses/geog493f08/Students/VTeeters.pdf

http://www.mundi.net/locus/locus_008/http://www.mundi.net/locus/locus_008/

http://www.geodetic.com/Whatis.htmhttp://www.geodetic.com/Whatis.htm

http://geospatial.overwatch.com/White_Papers_PDF/Multi%20Sensor.pdfhttp://geospatial.overwatch.com/White_Papers_PDF/Multi%20Sensor.pdf

http://www.cas.sc.edu/geog/rslab/index.htmlhttp://www.cas.sc.edu/geog/rslab/index.html- Good site with exercises created by J. Jensen, USC- Good site with exercises created by J. Jensen, USC

http://depts.washington.edu/rsgal/http://depts.washington.edu/rsgal/

http://seamless.usgs.gov/index.phphttp://seamless.usgs.gov/index.php

http://www.asprs.org/publications/pers/98journal/april/1998_apr_329-333.pdfhttp://www.asprs.org/publications/pers/98journal/april/1998_apr_329-333.pdf

orthorectification and triangulation

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