Geometric Potential of IKONOS Images

I. Preliminary analyses proved that the very main factor determining accuracy of orthoadjustment is a number, distribution and quality of photopoints used for leveling in adjustment process (so-called GCP – Ground Control Points). The goal of this experiment was to obtain an answer to this question: how a number of GCP and application of a given adjustment method affects the accuracy of orthoadjustment process? Evaluation was done on ICP points.


Figure 5. Accuracy of IKONOS for Parametric and RFM approach.


Tabele 2 Comparison of RMS and maximum errors over 35 ICPs of parametric and RFM models computation with 10 GCPs


II. Besides the ground control points, the quality of NMT is a decisive factor for accuracy of images orthoadjustment achievable in practice. Practical influence of NMT was investigated by way of orthoadjustment of Ikonos image with the application of NMT of various altitude accuracy. The following tables and figures show the results of correction of Ikonos images obtained in environment of PCI Geomatica. The tables present the average and maximum errors of adjustment with the application of various NMT and various correction methods. These errors were evaluated at the control points. For the needs of correction, one used three types of NMT on the areas of minor ground leveling: DTEDo, SRTM and DTED 2. The first one is NMT of grid 1000x1000 m. with an average error 10 RMSz, the second one, 100x100 m. with its terror 5 m and the third one 25x25 m with error 3.5 m.

Tabele 3 Comparison of RMS and maximum errors over 30 ICPs of parametric and RFM approach computation with 9 GCPs



Figure 7. Accuracy of IKONOS for RFM a) and parametric b) approach.


III. In the framework of experiment investigating influence of inclination of optical system for accuracy of the image orthoadjustment process of the very high resolution, one used two IKONOS scenes. One of them could be characterized with inclination from nadir point by 10.5o and the second one by 43o. In order to generate orthophotomap in PCI environment .

Table 4 Comparasion of RMS and maximum errors over 15 ICPs of parametric and RFM approach computation with 10 GCPs, with 43 and 10.5 deg. off nadir.



Figure 8. Accuracy of IKONOS for RFM a) and parametric b) approach.


IV The selected areas are around 23.3 km long by 12.4 km with covering an area about 121 sq. km in the north – west part of Krakow. Normal collection azimuth and normal elevation angle of satellite was used for calculating convergent angle (see Table 1). Overlapping was 97.3%. In order to realize the process of 3D geopositioning , were processed of 31 GCP with an accuracy of about 10 cm planimetric and 20 cm vertical. The height of the ground points range from 244 to 429 m During the survey, the terrain point were documented with photographs, on which the terrain situation and survey position were visible. The process of determining coordinates future points to be used for correlation and for controlling 3D geopositioning accuracy. In each case we tried to ensure that the accuracy of GCP identification on the imagery was definitely below one pixel.

Figure 9 present a specification of acquired accuracy of generated DSM on a number of GCP points. Achieved accuracy was checked on control points (ICP), with did not taken part in the process of DSM generation.

Table 5 Comparison of RMS over up to 30 ICP of RFM Model



Figure 9. Accuracy of IKONOS Stereo Imagery.



Figure 10. IKONOS, pan sharpen dropped over DSM.


Conclusion
Satellites IKONOS can be characterized by their considerable mobility and flexibility of imaging in the framework of a single run in range of the receipt station ROC. Thanks to considerable capacity for inclinations of optical system, one can achieve image for the needs of stereoscopy, optimization of programming the imaging of large areas and where one expects considerable inclination. From altitude of 680 km it is possible to focus optical telescope on the selected area of ground with accuracy of the dozens of meters. The data achieved in RPC format enable for geometrical adjustment of IKONOS images for application on a level of 1: 10 000. The very basic conclusions from experiments conducted may be formulated in the following way:

  • The very basic factor influencing the value of residual geometrical deviations in orthophotomap generated from IKONOS images is the selection and accuracy of ground control points used (GCP). The GCP used should be selected very thoroughly, and in the process of orthoadjustment they should be very carefully measured and interpreted.
  • For flay areas, with a use of NMT of altitude accuracy 2-4 m (type DTED Level 2) and with a use of one GCP from DGPS survey (of planimetric accuracy some 20-40 cm) by orthoadjustment of IKONOS image with an application o polynomial method (RFM) one achieved an average error in range 1-1.5 m. It was also noted that with the use of one GCP only one achieved a considerable improvement of orthophotomap accuracy.
  • At the same time, with the application of parametrical model, for adjustment of this imaging on the same GCP, one found out that its is necessary to use even 9 GCP In order to achieve the same accuracy as it is achieved with the use of multinomial model.
  • Good results were also obtained with a use of GCP from topographic maps scaled 1:10 000. The main advantage of such approach is a considerably lower cost of achievement of GCP from maps as compared with the measurement executed with a use of GPS technology.
  • Numerical Terrain Model coming from SRTM is sufficient for orthoadjustment of the VHRS. Even for strongly corrugated areas, Numerical Terrain Model DTED Level 2 and SRTM produce similar results for both types of image and methods correction.
  • Despite a considerable increase of pixel dimensions in the case of imaging with considerable inclination (30º-45º) it is relatively easy to obtain orthophotomap of an average error some 2 m, both with a use of multinomial and parametrical models.
Orthoadjustment of imaging of large inclination requires a use of greater number of GCP (for both adjustment models).

  • Stereoscope images IKONOS may be an excellent source for DSM generation. In case where there is no option to be supported by GCP, one can obtain RMSz of accuracy 7-6 m. When supported by several GCP one can achieve considerable improvement using RFM and taking advantage of multinomial coefficients obtained from RPC.
Acknowledgments
The author thanks to Mr Sebastaian Rozycki for corection IKONOS stereo images and help with owerly IKONOS ower DSM.


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