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DSM, Ortho, GSD, Google Earth, Point Cloud & Mesh Models Explained For Your Aerial Workflows with References
Photogrammetry: the science of making reliable measurements by the use of photographs and especially aerial photographs (as in surveying) (1)
Digital Surface Model | Orthomosaic | Ground Sampling Distance | Point Cloud | Mesh model, using the same subject.
These have to be our top three questions from our new clients. By sharing these short explanations, they might answer some of your photogrammetry questions as well.
DSM (Digital Surface Model) is an elevation model that includes the tops of buildings, trees, power lines, and any other objects. Commonly this is seen as a canopy model and only ‘sees’ ground where there is nothing else over top of it (2)
We present here a collection of 98 2D images of an earthen dam in Northern California with a DSM heat map overlaid on top of Google earth. Blue is the lowest point and red the highest. You can clearly see where the water outlet for this dam is. Less noticeable is the uncontrolled spillway location. Hint: it is the title image of this document.
Ortho or orthoimage is an aerial photograph geometrically corrected (“orthorectified”) such that the scale is uniform: the photo has the same lack of distortion as a map. Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances, because it is an accurate representation of the Earth’s surface, having been adjusted for topographic relief, lens distortion, and camera tilt. (3)
Here is the same subject with an orthorectified image overlaid the latest image from Google Earth. As is the case so many times with this process, the accuracy is off. Google Earthimages are in the neighborhood of +/-30m (4), while aerial drone images from NAA are in the centimeter range typically (6). While Google Earth has great coverage for the ‘large’, the solution comes down to the high def camera on an aerial platform less than two hundred feet above the ground to get images such as the one above to cover the ‘small’.
Ground Sampling Distance (GSD) is the distance between two consecutive pixel centers measured on the ground. The bigger the value of the image GSD, the lower the spatial resolution of the image and the less visible details. The GSD is related to the flight height: the higher the altitude of the flight, the bigger the GSD value.(5)
The final image above has included: DSM and Orthro images over a Google Earth map that is geolocated. The listed partial sampling of facts below are taken directly from the generated quality report within Pix4D for this dam. Each completed project generates a report that in conjunction with a survey team produces amazing accuracy and imagery. The total time aloft to collect all of this the aerial data was less than fifteen minutes.(6)
- Average Ground Sampling Distance (GSD) 1.86 cm / 0.73 in
- Area Covered 0.074 km2 / 7.4129 ha / 0.03 sq. mi. / 18.3271 acres
- RMS Error [m] 0.415261 0.650096 1.075402
- Geolocation Orientational Variance RMS [degree] Omega 0.868 Phi 1.039 Kappa 2.454 (Geolocation RMS error of the orientation angles given by the difference between the initial and computed image orientation angles.)
- Image Coordinate System WGS84 (egm96)
- Output Coordinate System WGS 84 / UTM zone 10N (egm96)
- DSM and Orthomosaic Resolution 1 x GSD (1.86 [cm/pixel])
- Number of Generated Tiles 1 / Number of 3D Densified Points 9441467 / Average Density (per m 3) 437.92
A Point Cloud is the set of three-dimensional distributed points which result from a laser or photogrammetry scan. Each point cloud is calculated by matching several photographs (in the case of photogrammetry survey) so as it corresponds to a point of the surface of the surveyed object. The density of a point cloud is dependent on the quantity and quality of the photos taken, but also on the followed protocol of the shooting(7).
This point cloud still image above has five million points of the smallest resolution available, generating a ‘dense point cloud’ that can be 3D rotated in real time. Our internal followed protocols for all of our drone missions provide: safety, efficiency and proven results. Your second hint: the spillway is in the lower right of the point cloud image.
A Mesh Model is a surface that is constructed out of a set of polygons that are joined together by common edges. A polygon mesh is a collection of vertices, edges and faces that defines the shape of a polyhedral object in 3D computer graphics and solid modeling(8)
This is usually the final step. After the various consultants have poured over the shared copy of the point cloud, adjustments, additions and suggestions can be collaborated within the cloud. Our final contribution concludes with this ‘pretty picture’ used to tie in all the digital aspects of the project. Commonly this is the marketing showpiece and is often presented with an animated 3D fly-thru.
We welcome your comments and are happy to answer any questions you have on this sample photogrammetry workflow.
References:
(1)https://www.merriam-webster.com/dictionary/photogrammetry
(2)https://gis.stackexchange.com/questions/5701/what-is-the-difference-between-dem-dsm-and-dtm
(3) https://medium.com/new-farmer/what-is-an-orthomosaic-photo-11140c0601df
(4)https://dds.cr.usgs.gov/srtm/version2_1/Documentation/MIL-PDF-89020B.pdf
(5)https://support.pix4d.com/hc/en-us/articles/202559809-Ground-Sampling-Distance-GSD-
(6)Pix4D excerpts of quality control report for the Francis Dam, data processed via NewAgeProcessing on March 31, 2018
(7)https://www.igi-global.com/dictionary/point-cloud/36879
(8)https://www.slideshare.net/foofiM/polygon-mesh