European Organization for Experimental ... - Host Ireland

Since this project was initiated it has become possible for users to ortho-rectify IKONOS
imagery themselves using the methods outlined below.
As reported by Thiery Toutin and Philip Cheng (Toutin and Cheng, 2000), the rectification
of IKONOS images can be performed using several techniques – a simple polynomial
method, the rational polynomial method, or the rigorous (or parametric) model method. The
simple polynomial method only takes into account the planimetric distortions at each ground
control point, and makes no allowances for the terrain. The rational polynomial method
uses a ratio of polynomial functions, together with a DTM of the ground, to produce a more
accurate rectification. The parametric approach takes into account the viewing geometry,
the terrain, the satellite position and orientation, and the sensor model, to derive an accurate
solution using a small number of control points. Toutin and Cheng (2000) show how a
rigorous approach can be used without the full IKONOS sensor model and viewing geometry.
Their technique uses the IKONOS imagery and its associated metadata to derive an
approximation of the viewing geometry and the sensor characteristics. These, together with
a set of ground control points, are then used within an orthorectification software package
to produce a rectified image. Results show that the process generates images which are of
comparable accuracy to those of the IKONOS Precision product. The report concludes that
the process gives users with access to accurate ground control points the ability to orthorectify
their own “Geo product” IKONOS imagery, at a much lower cost than the equivalent
IKONOS “Precision product”.
(Note: A similar methodology to the above is detailed by Karsten Jacobsen (University of
Hannover) in Annexe 3 of this report.)
A press release from PCI Geomatics
(http://www.pcigeomatics.com/pressnews/2001pci_si.htm) of June 18 th 2001, states that
there has been a large customer demand to have the ability to ortho-rectify the imagery
themselves. This has led Space Imaging to release a new product - 'Geo Ortho Kit' -which
allows the photogrammetric community to get more value out of their Geo Product. The
Geo Ortho Kit comprises the base image product (Geo Product) and Image Geometry Model
(IGM) file. The IGM is described as “a mathematical way of expressing the complex sensor
model of IKONOS which is required to correct the image for terrain distortions”. The Geo
Ortho Kit, together with the user’s own ground control points and a suitable DTM, will
enable customers to orthorectify the imagery to a high level of accuracy.
Note: since the time of this research, several other vendors have released software enhancements
enabling IKONOS Geo products to be orthorectified. These include products from
Leica Geosystems (ERDAS Imagine and SOCET SET), Z/I Imaging (ImageStation) and
Earth Resource Mapping (ER Mapper).
3.3 Geometric accuracy
As the solutions detailed in section 3.2 were not available at the time of the project, it was
decided in this trial to initially ‘rubber sheet’ the image to map detail using 2nd order polynomial
functions. A total of 40 points were used in each area. Ground control was in the
form of the 1:25,000 scale raster map. The accuracy of these ground control points is not
precisely known, but is presumed to be in the region of 8.0m RMSE. Table 3 below shows
the differences between each product using the raster map as ground control. Also provided
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