Guernsey Press

How do we arrange to get aerial images which are used to measure our properties?

AS I come to terms with my TRP bill I was wondering how Guernsey arranges to have a new satellite image every three years.


My wondering was provoked while looking at the latest Google map of the island and noticing that there are lots of small white clouds which actually prevent aerial inspection of certain properties.

Do we commission time on a satellite? Do we share Google’s satellite?

I find I am woefully ignorant of how we arrange to get the aerial images which are used to measure our properties (including roof overhangs) and which result in a significant amount of revenue flowing into States coffers.


St Andrew’s

Response provided by the States of Guernsey Cadastre:

Thank you for the opportunity to respond to your reader’s letter, and we are very happy to explain the method used to produce the high accuracy digital maps.

The initial digital mapping used for the introduction of TRP in 2008 involved low-level, high-resolution aerial photographs. These photographs were then mosaicked together and, using a process called photogrammetry, a new map of all the buildings in Guernsey and Alderney was created. These building shapes accurately represent the plan view area of each building element.

Since 2008 all new buildings including extensions, have been surveyed by our survey team using modern GPS and laser surveying techniques, making the islands’ mapping more accurate with each passing year. We are not aware of any cases so far where there have been building measurement errors that fall outside of the parameters set out in the TRP legislation. Changes to the size of buildings result in revised assessments and TRP bills being calculated by the Cadastre.

Additionally, every three years, aerial photography is specially commissioned, involving an aircraft fitted with a high-resolution cameras performing a grid sweep of the Bailiwick. The resulting photographs are processed and mosaicked to produce a single image of the island. This image is then overlaid on the digital mapping to check that the mapping is up to date and accurate. The flyover is always performed on a clear day and usually during the early summer months. This is next due to take place in 2025. Satellite imagery is not used as part of this process.

We hope that helps to clarify how this process works, and gives reassurance that it is a very effective process for producing and updating reliable images to a high degree of accuracy.