Lab 3: Processing & Understanding Data Using Pix4D
Jack Radenz
Introduction:
Data that was collected during lab 2 was pre-processed in Pix4D to create outputs (hillshade, DSM, orthomosaic, etc.) to be products that could be useful to many forms of geospatial analyzing of the physical environment. While the outputs created in lab 3 do not help the team of students answer a scientific question, it helps the students understand these outputs and how they may be useful products in future study or work.
Data Collection:
See UAS Lab 2 for details.Methods:
See UAS Lab 2 for details.Discussion:
1). It is necessary to have a high amount of overlap during UAS data collection for mapping applications for mainly two reasons. The first reason is because the UAS is legally obligated to fly 400 feet AGL or less. Therefore, the UAS is normally flying at low altitudes. It needs to have a high amount of overlap so keypoints can be generated between images. If images are taken too far apart, keypoints will not be able to be extracted. Secondly, short range photogrammetry (UAS data collection) utilizes a cheap, inexpensive camera. Therefore, the pitch, roll, and yaw (camera parameters) are not as accurately calculated for each image. The high amount of overlap helps the camera compensate for the inaccuracy in parameter calculation.
2). A DSM is an elevation model that accounts for everything that exists on the surface of the earth (trees, buildings, vehicles, etc.). While the DTM is an elevation model that removes trees, buildings, and everything that is a sudden elevation change on the surface of the earth.
Figure 1. DSM of Myrick Park, La Crosse, WI
3. The largest difference between a DSM and TIN is being that a DSM is a raster output and a TIN is a vector output. TINs can be created by lines, points, and polygons. TIN creates an elevation output by connecting lines to equal points of elevation. DSM is created by pixels which hold a specific elevation value.
4. A georeferenced mosaic is assuming that the earth is 2D. The georeferenced mosaic does not take into account the elevation changes or the 'roughness & rigidness' of the earth. An orthorectified accounts for the vertical, 3D dimension (roughness & rigidness) of the earth. A only georeferenced mosaic of the earth is much less accurate than an orthorectified. For example, if a person were to walk linearly 2 miles west on a flat surface or on a hill they would cover more ground horizontally on a flat surface (georeferenced). Georeferencing portrays an inaccurate portrayal of the earth.
5. The descriptive statistics of a DSM would be mean, median and mode. We can use these in many ways to understand the landform without even needing to look at it. The mean will give the surveyor a general idea of how high above/below the land exists relative to mean sea level. A high mean would generally imply an area of high general elevation (i.e. Himalayan Mountains). The median gives the surveyor an idea of steepness of the terrain. If there is a large discrepency between the median and the minimum and maximum values it would imply that the terrain is very steep in that area. The mode really helps the surveyor understand what kind of land they are surveying. For example, if the mode was 700 ft. and there were numbers in your data that went from 701 - 1,400 ft. the surveyor could assume they are surveying a canyon floor. The mode allows the surveyor to see values of which the most land resides on.
6. Within the DSM collected at Myrick Park the surveyor can see the DSM go from low values in the north to high values in the south. This is a uniform distribution of values as the entire output become larger values as the land moves south. The surveyor can assume that a hill is moving from south to north which forms into wetlands or low lying area on the north-most end of the study area.
7. Some patterns that exist in the orthomosaic of Myrick Park is that the shadows are all towards the north end of the trees and structures. Therefore, the sun must have been directly in the south sky and the images must have been acquired around noon. The grass and vegetation on the slope is much lighter than the vegetation on the flat areas. This is because the slope has a north facing aspect and fails to capture as much sun as other areas.
8.The hillshade map was produced from the Myrick Park DSM. The DSM contains raster cells of elevation values. Within the hillshade tool, the analyst is able to tell ArcMap where the sun is located in the sky and its angle. That is then applied to the DSM. The DSM raster cells communicate their elevation values with the rest of the pixels in the DSM. Because the DSM has both spatial location and elevation data, it is able to determine the where shadows will exist, and the length of those shadows.
4. A georeferenced mosaic is assuming that the earth is 2D. The georeferenced mosaic does not take into account the elevation changes or the 'roughness & rigidness' of the earth. An orthorectified accounts for the vertical, 3D dimension (roughness & rigidness) of the earth. A only georeferenced mosaic of the earth is much less accurate than an orthorectified. For example, if a person were to walk linearly 2 miles west on a flat surface or on a hill they would cover more ground horizontally on a flat surface (georeferenced). Georeferencing portrays an inaccurate portrayal of the earth.
Figure 2. Orthomosaic of Myrick Park, La Crosse, WI
5. The descriptive statistics of a DSM would be mean, median and mode. We can use these in many ways to understand the landform without even needing to look at it. The mean will give the surveyor a general idea of how high above/below the land exists relative to mean sea level. A high mean would generally imply an area of high general elevation (i.e. Himalayan Mountains). The median gives the surveyor an idea of steepness of the terrain. If there is a large discrepency between the median and the minimum and maximum values it would imply that the terrain is very steep in that area. The mode really helps the surveyor understand what kind of land they are surveying. For example, if the mode was 700 ft. and there were numbers in your data that went from 701 - 1,400 ft. the surveyor could assume they are surveying a canyon floor. The mode allows the surveyor to see values of which the most land resides on.
6. Within the DSM collected at Myrick Park the surveyor can see the DSM go from low values in the north to high values in the south. This is a uniform distribution of values as the entire output become larger values as the land moves south. The surveyor can assume that a hill is moving from south to north which forms into wetlands or low lying area on the north-most end of the study area.
7. Some patterns that exist in the orthomosaic of Myrick Park is that the shadows are all towards the north end of the trees and structures. Therefore, the sun must have been directly in the south sky and the images must have been acquired around noon. The grass and vegetation on the slope is much lighter than the vegetation on the flat areas. This is because the slope has a north facing aspect and fails to capture as much sun as other areas.
8.The hillshade map was produced from the Myrick Park DSM. The DSM contains raster cells of elevation values. Within the hillshade tool, the analyst is able to tell ArcMap where the sun is located in the sky and its angle. That is then applied to the DSM. The DSM raster cells communicate their elevation values with the rest of the pixels in the DSM. Because the DSM has both spatial location and elevation data, it is able to determine the where shadows will exist, and the length of those shadows.
Figure 3. Hillshade map of Myrick Park, La Crosse, WI at 4:00 pm