Getting to know Pix4D
Pix4D is a suite of photogrammetry software for drone mapping. The software boasts an enormous selection of features that aim to create highly accurate 2D maps and 3D models. Pix4D works flawlessly with many modern UAV’s, by combining the pictures taken with a UAV, they can be converted into a number of different models and surfaces that can provide enormous potential for projects world wide. This article aims to explain some of the terminology used within the software and its application.
Within a 3D model a point cloud is a collection of 3D points that provide a foundation for precise measurement and model rendering. The density of these points determines the accuracy of the model.
A 3D mesh is an interactive model of the surveyed data and created by matching colour triangles within the point cloud. Often this view is the best way to showcase the model due to the highly realistic nature of the rendering.
Digital Terrain and Surface Models
As both of these models look very similar to the untrained eye, it should be noted that they both have different applications and uses depending on the individual using them. A Digital Terrain Model (DTM) illustrates the natural landform, highlighting all depressions and elevations without the inclusion of man made features. A Digital Surface Model (DSM) highlights the surface topography with both natural and manmade features. Both of these features are displayed as a 2D image with a colour graded scale to make identifying the range of depth easier.
An Orthomosaic combines all pictures taken throughout the survey to create one geometrically corrected image.
Unlike a standard aerial photograph, an orthomosaic corrects a number of different distortion factors including the earth’s surface. The resolution is significantly higher than a traditional aerial photograph and the image is scaled correctly, allowing measurements to be taken from it if desired.
The image above shows an orthomosaic overlaid onto a standard satellite picture. The significant improvement in detail is clear at this zoom level, but we can attempt to demonstrate the achievable detail by drawing your eye to the area highlighted in blue.
Below is a magnified section taken from the same image, zoomed into the area in blue, which shows the vast improvement in detail that can be achieved through the orthomosaic.
Within the Pix4D cloud based service you will find a tool that allows for an elevation profile to be generated. Elevation profiles can only be generated after a 3D model had been created and uploaded – UAV 365 will take care of this part on your behalf.
The elevation profile can be created between any two points in any direction within the model. This feature allows multiple elevation profiles to be taken over the surveyed area, which often makes for much easier further analysis.
Ground Sampling Distance
The Ground Sampling Distance (GSD) is a term often associated with the resolution and accuracy of a data set. The distance between the centre of two pixels is defined as the GSD. If the spacing between pixels is small, resolution is subsequently greater and this therefore improves accuracy. The distance from the ground from which the picture is taken is often a good indicator of how big or small the GSD will be and therefore how accurate the model will be.