Click here to view latest available 0.5M and 0.25m GSD imagery from 2008 onwards
In 2005, National Geo-spatial Information noticed a global trend towards digital image acquisition and decided to invest in a digital camera (an Intergraph DMC). Since 2008 -2016, all images have been captured digitally with this camera as well as similar contracted cameras. Since 2017 all imagery is now being captured at 0.25m GSD with new generation cameras providing unprecedented detail and clarity.
This meant that the acquisition of the traditional photo-scale had now been replaced with a Ground Sample Distance (GSD). The ground sample distance is the size of 1 pixel on the ground and is influenced by the flying height and focal length.
Currently, 12-bit images are captured in RGB (true colour), Near infra-red and Panchromatic, with a GSD of 0.25m. They are stored as Tiff files with a JPEG compression of Q=3; with tiles and a full set of overviews. Each DMC image is approximately 7km by 3.8km and they are mosaicked together to fit the 1:10 000 reference sheets after orthorectification. The aim is to capture 40% of the country every 3 years and the remaining areas every 5 years.
Images are captured in RGB and CIR as can be seen from this Cape Town image captured in 2008.
Please click here for updated ortho-rectified imagery
Orthorectification is the process whereby images are corrected for all the distortions present within that image. Distortions occur because of displacements due to variations in the terrain, as well as earth curvature and the varying scale from the centre of the image to the outer edge.
The orthorectification process requires the aerial triangulation file and a digital elevation model (DEM). The aerial triangulation file contains a list of the coordinates and orientation of the digital camera at each exposure and the DEM is a model detailing ground heights in the image. The accuracy of the final orthophoto is largely dependent on the accuracy of the DEM (See image below courtesy of PCI Geomatica)
Once the image has been orthorectified, measurements can be accurately taken from the image. Coordinates can also be read directly from the image. The accuracy of NGI's orthorectified imagery is 3metres at the 2 confidence level.
Below: The accuracy of the orthorectified image is dependent on the accuracy of the DEM (Picture courtesy of PCI Geomatica)