What if a drone can't be used for aerial measurements?
Last year, we were asked by a client to provide some aerial imagery to help with a query regarding the positions of some elements of a construction site compared to the positions stated in a planning application.
The area concerned appeared to be spread across a couple of kilometres squared with the only space suitable for Take off/Landing (TOAL) appeared to be a couple of Byways Open to All Traffic (BOAT) which weren’t ideal for use, plus the distance from these locations would have meant that the 500m Visual Line Of Sight limit would not be sufficient for imaging the required locations.
Looking at Google Earth, Google maps and Bing Maps, it appeared that some stage of the construction was visible, though not the parts that the client was interested in. They had, recently, had the opportunity to take some aerial photographs from an aircraft, but being far from NADIR images it wasn’t really possible to garner any useful accurate measurements from them.
We suggested purchasing some current, or very recently acquired, satellite imagery of the area, something that has been more commonly available in the recent past. Although nowhere near the current cm level accuracy available from drones (at least in lower cost satellite images!) it could be possible to obtain, for a reasonable cost, fairly recent images of the area in question, with accuracy somewhere close to 300-500cm/pixel, which would be good enough for their needs.
Doing an internet search, several providers of satellite imagery were found, we settled on www.spymesat.com as it seemed to be one of the easiest to navigate, and seemed to have a lot of options for images. It would be possible to ‘order’ an image from a subsequent ‘flyover’, but at a cost of possibly in excess of $1000, and the probability that cloud cover could obscure some of the required locations, we did a search of the location to see any images in their archives.
Eventually, we found a fairly recent image that had little cloud cover, and the locations required, although from a few weeks earlier, it appeared to have the locations of interest shown. Measuring the extents of the site using google maps, it was close to 2km x 2km, so the equivalent GEOTIFF image was selected using the approximate latitude and longitude of the centre of the site. This was then purchased and downloaded.
As a basic ‘sanity check’ the GEOTIFF was imported into Google Earth to ensure that any landmarks, roads and tracks etc on the the edges of the image coincided with the corresponding landmarks, roads and tracks on Google Earth. As expected it all appeared to line up after checking for details along the edges of the geotiff lining up exactly against the older image in Google Earth.We were then able to continue to find the latitude and longitude of the parts of the site that were of interest.
The GEOTIFF was then imported into a Linux instance of QGIS (https://www.qgis.org/en/site/) as a new raster layer.
In order to precisely find the location of each item of interest, it was necessary to zoom into the image, and select the approximate centre of each item of interest. clicking the mouse at each location displayed the latitude and longitude which were noted.
Since the locations in the planning notice were quoted in ‘Eastings and Northings’ the latitude and longitude were converted using the online tool provided by the British Geological Survey (https://webapps.bgs.ac.uk/data/webservices/convertForm.cfm) and reported as such to client.
If you have any need for aerial or drone imagery, or help in areas where access for drone imaging could be difficult, by all means contact us.
Alternatively, give us a call on (01584) 517070