I'm trying to create orthomosaics with accurate measurements of rocks in a stream bed. All the photos were taken from a fixed height and scale bars were added by knowing the dimensions of GCP's along the stream. However once the orthomosaic is built, the ruler tool shows that the size of the GCP's is innacurate (ie 1.7m instead of 9cm). Furthermore if you update the scale bars they all congregate in a random area of the stream bed and while still in order are significantly closer together.

Does any one know why this is happening?

We found that if we update the scale bars every time we add a new scale bar then build the orthomosaic, then this doesn't happen (though we have no idea why).

Is there a easier way to fix this?

Having to go back and update the scale bars before building the orthomosaic would take a huge amount of time.

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