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The Ground Sampling Distance (GSD) is the distance between two consecutive pixel centers measured on the ground (usually cm or inches). A smaller GSD implies a higher resolution and more detail, but also a larger image (in pixels, and therefore also in file size). Larger images also take more time to be computed.
Since Pix4Dfields 1.6 the user can specify the desired GSD of the computed orthomosaic, as described below.
In Pix4Dfields 1.5 and previous versions the size of the orthomosaic was capped at 50 megapixels; in Pix4Dfields 1.6 the user can set a different value or disable the limit.
The GSD of the generated orthomosaic is determined by specifying one or both of the following settings in the advanced processing options:
- Minimum GSD (default: 0.0).
- Maximum total megapixels (default: 50 MPx).
The GSD of the generated orthomosaic is chosen such that it is:
- as small (high-resolution) as possible, but
- not smaller than the specified Minimum GSD, and
- not smaller than the approximate length covered by a pixel in the input images, and
- the number of generated pixels does not exceed the specified Maximum total megapixels.
The default values are chosen such that Pix4Dfields 1.6 behaves the same as 1.5 and before.
Please be aware that the required disk space increases with increasing number of input images and decreasing GSD.
Since version 1.6 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 harddrive.
Images that are wider than 32678 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.
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).
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.