If geotagged images are used and the volume measurements are supposed to be computed for small areas, GCPs are not required for accurate volume measurements. However, if volumes have to be calculated for large areas, GCPs are required for accurate measurements.
When calculating the volumes, the vertices of a base have to be marked in at least two images in order to calculate the error. Additionally, it is important to select the right settings for the base that fit your purpose. Step by step instructions how to draw a volume can be found in the following article 202560319 and description of settings can be found here 208652926.
If the same base is used in both Pix4Dmapper and Global Mapper for volume measurement, the difference in values should be minor. It is possible to export the base from Pix4Dmapper which can be imported to Global Mapper. For more information: 211245943.
To achieve accurate volume measurements we recommend to have high overlap between images, sharp images, well distributed GCPs, Manual Tie Points (MTPs), etc.
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Additionally, please note the Trial version has full capabilities of the software.