1. Using Photogrammetry to Generate a DEM and Orthophoto
Prepared by: Keith Blonquist
For: CEE 6440

2. What is photogrammetry?
**Photogrammetry is the science of using images to obtain information about physical objects
**Geometrical aspects of photogrammetry (recovery of 3D geometry)
What is photogrammetry used for?
**Photogrammetry is used extensively to collect data for GIS databases
**Photogrammetry involves aerial images, satellite images, etc…
**Most GIS databases contain data-sets that were collected using photogrammetry
What does this presentation cover?
**Generation of a various DEMs and an orthophoto from aerial images
**This will be demonstrated with both a synthetic and actual data-set

3. Principles of photogrammetry
Perspective projection: from 3D to 2D
Collinearity equations: express 2D image coordinates (x, y) as a function of the 3D coordinates (X, Y, Z) and camera orientations
Multiple images: a point must be visible in 2 separate images in order to find its 3D coordinate
Bundle Triangulation (based on collinearity equations) simultaneously solves for camera orientations and (X, Y, Z) coordinates of target points from image coordinates (x, y) of point correspondences

4. Synthetic Data-set Synthetic terrain Synthetic images
Point correspondences are corresponding points located in several images

5. Resulting 3D points from Bundle Triangulation (in an arbitrary coordinate system)
Using ground control points to orient model to known coordinate system
Ground Control points are points for which the (X, Y, Z) coordinates are known in the desired coordinate system

6. Import 3D points and polygons to ArcGIS
Points interpolated to TIN (points on tops of house were clipped out)
Triangulation done by Delaunay triangulation—you can specify break lines if you wish
TIN converted to raster

7. Elevation Interpolations
Kriging
Inverse Distance Weighting
Splines
(points on tops of houses were clipped out)

8. Orthophoto vs. Actual Image
Orthophotos are a result of parallel projection 
Actual images are a result of perspective projection
Actual image vs. Orthophoto

9. Actual Dataset
Images taken with a Wild RC30 aerial camera. Krawietz Aerial Photography, San Antonio, TX

10. Selection of Point Correspondences
***This collection of point correspondences is fairly sparse—so the resulting DEMs will not be as accurate as possible. More accurate models result from more dense point collection. In practice, automatic point correlation or use of stereo plotters can be used to more densely collect points.

11. Orientation from ground control points
Resulting points
3D point cloud from Bundle Triangulation in arbitrary coordinate system
Orientation from ground control points
Coordinates of control points in
North American Datum 1983, Texas State Plane Coordinate System, South Central Zone
North American Vertical Datum of 1988 in U.S. Survey Feet
ID X Y Z

12. Precision of Solution:
**Bundle Triangulation Algorithm gives error estimates of points in 3D space
**For these two images the 1-sigma error in 3D space was 0.43 ft (13 cm)
**This means that there is a 68% probability that the calculated points are within 0.43 ft (13 cm) of the actual points, and a 95% probability that the calculated points are within 0.86 ft (26 cm) of the actual points
**A higher number of points can increase this precision (more accurate camera positions are calculated)
**More accurate camera models can also provide higher precision (these calculations did not involve corrections for lens distortion)
**RMSE for transform to Texas State Plane Coordinate System = 0.73 ft (22 cm)

13. Import point and polygon files to ArcGIS
One point file, one polyline file (break lines) and one polygon file (bridge)

14. Generation of TIN and Raster

15. Splines
IDW
Kriging

16. Work still to be completed:
Summary:
**Photogrammetry can be used to generate accurate DEMs and Orthophotos
**Accuracy and detail of models is highly dependent upon the number of point correspondences that are chosen
**Some features (houses, bridges) can be problematic
Work still to be completed:
**Still need to generate an orthophoto from the actual images

17. Questions?