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The Brief

Horton Levi Ltd carried out an aerial heat-loss survey on behalf of Chester-le-Street District Council.  In order to make best use of the data, the Council required a geo-rectified image map capable of overlaying their existing map/landline data and aerial imagery.  In this instance, the end product had to be compatible with CadCorp, OSMastermap and Mr. Sid imagery files.


Stage 1 - data collation

We were provided with just under 2,300 individual thermal image files by Horton Levi in bitmap (.bmp) format, together with landline and sample imagery data.  The District area is largely rural, but contains the towns and villages of Chester-le-Street, Great Lumley, Sacriston, Pelton, Oulston, Bournmoor, Kimblesworth and Edmondsley.  Our aim was to georectify and mosaic the entire district to create a seamless map - the total area amounted to approximately 80km˛.


Stage 2 - data formatting

The landline data was provided in OSMastermap compressed .gz format.  We extracted this to .gml format and then converted the entire map to a shape file for georectification purposes.

Likewise, we converted the thermal data from .bmp to .tif, cropped each image and approximately re-orientated them to North where necessary.


Stage 3 - georectification of aerial data

Each image was individually rectified.  In practice, this means that reference points, known as Ground Control Points (GCPs) were plotted on each image and then their precise matching point was plotted on the map data.  We estimate that approximately 21,000 GCPs were plotted in total - averaging 262 per 1km˛.  This ensured remarkable levels of accuracy in Stage 4.

Due to the rural nature of the survey area, large parts of the map were fields and farmland, much of which is difficult - and at times impossible - to rectify against map data.  However we developed a system allowing us to do just this, ensuring that there was not a single gap in the data.

Stage 4 - Constructing the final data map

With all the data rectified, we began the task of producing a complete data map.  Each image is stitched and blended together, producing a seamless mosaic of the entire area. 
The data was projected using British National Grid co-ordinates so that it would precisely overlay both the landline data and aerial photography. 



Stage 5 - Presentation of Data

The completed product was provided in three formats on DVD:
  • GIS compatible to form an additional layer in the Council's mapping system
  • GIF and JPG format for global viewing and presentation purposes
  • Individual image files in original greyscale and colourised for heat-loss analysis


We join together aerial photographs to form a seamless image


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