Automatic Georeferencing (AGR) Best Practices
Automatic Georeferencing capability is based
primarily upon the ability to match features in images between two flight
instances surveying the same site.
In order to filter out false positives, the
matching criteria are very strict. For this reason, AGR will not work well when
there have been significant changes in the terrain between the two flights.
AGR will work best in the following case:
- The area will be surveyed frequently. Frequently
here is relative to the pace of work in the area. If many changes are made on a daily basis, it
may be necessary to take a survey once a week or more, but if the work is going
at a slower pace, it may be sufficient to do one survey instance a month.
- First flight instance is successfully and very
precisely geo-referenced using a good set of GCP’s. This instance will precede
and will be the base of the succeeding automatically georeferenced flight instances. Please refer to GCP Distribution in the Survey Area article for more information.
- The surveyed area shape is the same along the life
of the project. If new areas are added
and the flight plan is changed to include the new areas, it may be necessary to
do the first flight instance that includes the new areas after marking a new set of GCP’s in that area in addition to renewing the GCP’s in the old area. The whole new set of GCP’s should be used to
geo-reference the whole work site again, which will be used as the base for
later automatically georeferenced flight instances.
- Stick with the same camera/drone type throughout
the project. Using images taken with
the same camera/drone types gives a higher success rate in auto georeferencing.
AGR will not be suitable in the following cases:
- Significant construction work has been performed
in the entire site. For example, leveling of the entire site or removal of all
- A long time has elapsed, and the terrain has
changed due to seasonal factors such as snow, rain, color and quantity of
- Significant change in the altitude between the two
- Significant change in the flight plan between the
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