Orthomosaic GSD - PIX4Dfields

Orthomosaic GSD

The Ground Sampling Distance (GSD) is the distance between two consecutive pixel centers as measured on the ground. Typically the units are in centimeters or inches. When the GSD becomes very small and the resolution is very high, the size of the files (bytes) can become very large. These large files also require more time for processing. With PIX4Dfields it is possible to specify the desired GSD of the outputs in order to produce smaller file sizes that are easier to work with. The steps are described below.

Specifying the GSD in the processing options menu

The GSD of the outputs can be selected under Resolution in the Processing options menu. A slider allows selecting the desired GSD, ranging from Full resolution to Low resolution. The GSD will be shown in either cm/pixel or in/pixel depending on the units selected in the dashboard settings. The file size and the processing time are estimated.

Processing options menu.

Viewing orthomosaics

PIX4Dfields supports generating and viewing images that are larger than the computer’s main memory. PIX4Dfields ensures that images can be viewed fluidly. It does this by computing higher GSD (downsampled) versions of the image, called overviews. When zooming, the appropriate version is displayed automatically. When panning or zooming, a small delay maybe noticeable as data is fetched from the hard drive.

Images that are wider than 32,678 pixels are never displayed at the full resolution for performance reasons. When zoomed in completely, they are displayed at 2x the GSD of the full-resolution image. The full-resolution image is stored on disk, however. To view it at highest resolution you may use the Export function and open it in an external viewer, such as QGIS or ArcGIS.

What is the maximum orthomosaic size that I can compute with PIX4Dfields?

PIX4Dfields does not impose a limit on the number of input images and the size of the generated orthomosaic. The maximum orthomosaic size is determined by hardware limitations (disk space, memory size, operating system maximum file size) and time limitations (the time to compute an orthomosaic increases quickly beyond an input dataset size of several thousand images).

What are overview (.ovr) files?

To be able to display large images fluently, PIX4Dfields computes downsampled versions. These are stored in overview (.ovr) files in the PIX4Dfields data folder. PIX4Dfields automatically takes care of their creation and deletion; the user never has to work with them directly.

Overview files are generated automatically after an orthomosaic, a trimmed orthomosaic, or index has been computed. They are also created when GeoTIFFs are imported. A progress bar is displayed while computing the overview. Additional disk space is required for storing the overview files. Overview files are never exported or synchronized with the cloud.