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Photogrammetry is the science of making spatial measurement and other mathematical products from photographs. The word photogrammetry comes from three Greek words.
Photos = Light
Gramma = Letter or something drawn
Metrein = To measure
We use two types of photogrammetry, vertical & oblique.
To create 3D models of objects like a buildings or statues, we use oblique photogrammetry, this enables us to produce 3D models with detail on all sides. Vertical photogrammetry does capture buildings and objects quite well but can lack allot of detail, especially when you zoom in to ground level.
First we work out the best flight plan to capture the object, we then work out the time of the day each side of the object will be getting best light, some objects may take all day to capture.
We then place markers, a bit like GCP’s but these markers are for scale, we place two markers next to object to be captured and measure the distance between, we can then input these measurements when processing the data to create a scaled 3D model.
When the lighting is right we can start collecting data, we set the camera on the UAV facing down at a 45` angle, if the light allows us we can collect all data in one flight, we then either create waypoints or program UAV to circle object or fly manually. We then fly around above the highest part of the object capturing lots of overlapping images, then we reduce altitude and continue circling the object taking images, once low enough we adjust the angle of the camera up and continue circling and collecting images.
Once we have collected enough data we can then process the images to create a 3D model.We can then merge oblique photogrammetry models onto vertical photogrammetry models to create an overall detailed model.
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Vertical photogrammetry is what we use produce our Topographical maps, orthomosaic images and 3D models of areas. Picture A below gives you an idea of how we collect the data using ground control points (GCP’s) for more accuracy when processing the data from aerial serveys.
First we place uniquely marked GCP’s in the area to be surveyed, we then use a tool that uses the Global positioning system (GPS) to give us the coordinates of each GCP. We then program the UAV to fly a pattern at set altitude above ground level (AGL) as well as desired image overlap, more overlap equal’s higher accuracy.
The altitude gives us the desired ground sample distance (GSD), lower the AGL gives each pixel a lower measurement in distance, higher AGL gives each pixel a higher measurement in distance, e.g. UAV survey height set to 40m AGL using 35mm lens gives us about 4.5cm distance per pixel GSD.
The UAV then locks on multiple satellites, the camera is positioned facing directly down, the UAV takes off and ascends to the set altitude to begin flying the pre-programmed flight pattern.
The UAV will fly the pattern collecting images at the correct times to achieve the desired overlaps as well as photographing the uniquely marked GCP’s, each image taken is given an electronic tag with geographical coordinates (GOEtag). Once the pattern is complete the UAV will return and land.
We then take all the information we have collected, load it into our system to create the desired product.