DIGITAL ORTHOPHOTO (Linnet - The Land Systems Company)
    
	Product Description
     
	 Some of the key technical solutions used for the production of Digital Orthophoto Files (DOI) include:
     · GPS controlled precision aerial photography by certified ICAS contractor
     · Cessna Conquest aircraft (8 hours operating time at 10,500m) for 1:60 000 scale photography acquisition
     · GPS ground control points by Geodetic quality receivers
     · Aerial Triangulation & GPS adjustment combined for highest precision
     · DEM collection with high precision analytical plotters
     · DOI production using state of the art software
     · DOI pixel size is 2m. Each file covers 100km2 (10k tiles)
     · All DOI tiles are concurrent and edge tied to within +/- 2 pixels.
	 · All DOI files delivered as geo-referenced TIFF format
	 
    Methodology
     
	 Aerial photography
      DOI production is from 1:60000 Black & White aerial photography acquired by North West Geomatics of Edmonton.  North West has a fleet of aerial survey aircraft equipped with the latest technology in large format Leica/Wild RC30 GPS aerial survey cameras. The camera was equipped with forward motion compensation and is current state of the art technology. An automatic exposure meter provided continuous exposure settings compensating for the combined effects of terrain and cloud coverage. Photography acquisition was controlled by an on board flight management system that provides precise GPS co-ordinates for each photo centre. Photos were taken at precise intervals to correspond to the edges and centre of each DOI 10 km file. These data were later used in the DOI production. 
     
	 GPS Ground Control
      Pollock & Wright Land Surveyors of Winnipeg, Manitoba provided the ground control positions to reference the GPS controlled aerial photography. Pollock & Wright established horizontal and vertical control at locations specified for the aerial triangulation block adjustment.  The points were established relative to the existing Geodetic Control Framework and are based on the NAD'83 horizontal datum and prevailing Provincial Vertical control network. All of the points were photo identifiable points selected from newly acquired photography.
      Control was established using Global Positioning System (GPS) measurements.  Four Trimble 4000SSE dual frequency GPS receivers were used for all GPS measurements.  Dual frequency carrier phase measurements were carried out using GPS static techniques in accordance with the Guidelines and Specifications for GPS Surveys as published by the Geodetic Survey Division of Natural Resources Canada.  
     
	 Aerial Triangulation
      The prints and diapositives produced from the 1:60000 scale aerial photography were prepared for triangulation by identifying and pug transferring seven photo point identifiers per full frame of photography.  This provided a total of five common model tie points between each stereo model and four common line tie points per stereo model between each line of photography.  These tie point positions were identified with a 60-micron drill hole in the emulsion of the film. The measured plate co-ordinates were processed along with the GPS camera data and the ground control survey data to produce a final ground adjustment file for the entire aerial photography block.
      The final aerial triangulation adjustment was completed with the rigorous least squares bundle adjustment software GIANT (General Integrated Analytical Triangulation).  GIANT is a general-purpose bundle adjustment designed for large photogrammetric blocks utilizing ground and/or airborne control.  
     
	 DOI Production
      From the 1:60000 scale aerial photography topographic data was compiled with a Leica SD2000 analytical instrument and a KLT DSP digital stereo image workstation. Data were collected in the form of a 'DEM' (digital elevation model) file, consisting of a dense network of random three dimensional point data (compiled at an approximate 100-120 metre grid spacing).  As well, breaklines were compiled along features such shorelines, streams, roads, railways and other natural extreme deflections in the terrain. 
      For each 10km x 10km UTM grid file the centre diapositive was scanned on the Vexcel 3000 precision scanner, at a resolution of 30 microns.  The ground control data produced from the aerial triangulation was used to geo-reference the digital image.  The 'DEM' file containing the three dimensional point data is referenced to the digital image file, then processed digitally rectifying the image, removing all distortions and scale differences throughout the photograph. The digital ortho image has a final ground pixel size of 2 metres. 
     
	 Availability
      The digital orthophotos in TIFF format, complete with a geo-reference file (TWF) and elevation data (DEM) for each ortho image is available from Linnet - The Land Systems Company. All data is available on CD ROM.
