Investigation of effect of the number of ground control points and distribution on adjustment at WorldView-2 Stereo images

Nowadays, Very High Resolution Satellite (VHRS) images have been used for many applications intensively. Using of the digital satellite images in relevant approaches may give more accurate ideas about the researched fields. Besides, different methods could be used for production of maps from relevant digital data in many scientific disciplines. One of these methods is to use VHRS images. The most importance reason of using the maps, which are derived from VHRS images, is accuracy of data. To obtain maps from VHRS images or use VHRS images as map, the VHRS images required rectification. For a accurate rectification process, Ground Control Points (GCPs) must be marked in the field. In this study the effect of number of GCP and distrubition on the surface has been investigated on stereo WorldView-2 (WV-2) at rectification of VHRS images. The effect of distribution of control points at the terrain has seen to be more effective than the effect of number of control points upon the adjustment results.


Introduction
High resolution satellite images, as less than 1 m. resolution are used in many fields. For example, updating existing maps, land use mapping, urban planning, disaster monitoring, and so on. To use high resolution satellite images in such fields, they had to be rectified. Rectification process is done by points, which coordinates are known in both land and image. The selected points may indicate either measured points in the field or/and can be designated as visible points in images. Sometimes, these points may represent; road cross, painting objects, etc. Points which are set up in the field are either acquired from existing maps or from image matching the research site or geodetic surveys. In this study the effect of number of GCP and distrubition on the surface has been investigated on stereo WorldView-2 (WV-2) at rectification of VHRS images.

Importance of Control Points
Acquisition of ground control points (GCPs) is particularly importance for geometric correction of high resolution satellite images. The commercial high resolution satellite image can be accurately rectified using the combination of bias-corrected rational polynomial coefficients (RPCs) and ground control points (GCPs). With a few precisely measured GCPs accurate three dimensional measurements can be made from the base level Ikonos data product. However, the accuracy of the results is dependent on the precision of the GCPs [1]. There are two approaches in geometric correction of high resolution satellite images. One of them is the bias-correction procedure for rational polynomial coefficients (RPCs), which requires a minimum of only a single GCP, but it gives of course require RPCs. The other is affine model that requires a minimum of four GCPs per scene, though six as practical minimum would be recommended [2]. The control points could be acquired by three different methods for geometric correction in two and three dimensional of high resolution satellite images. These are; GCPs from existing maps, GCPs set up in land, GCPs from image matching. Kadota and Takagi had used above mentioned methods in acquisition of GCPs for geometric correction of high resolution satellite image. They had achieved to best result with surveyed GCPs [3]. Dial and Grodecki had tested Ikonos stereo accuracy without ground control. They had achieved to absolute accuracy at 6.2 m. horizontal, 10.1 m. vertical, and found out the most relative accuracy results for points more than 3 meters [4]. Hanley and Fraser had used ground surveyed GPS points and ground control measured from the orthomosaic for two dimensional geometric correction of high resolution satellite images. They had achieved to similar results from both surveyed GPS points and orthomosaic points [5]. Dare et. all. had used ground surveyed GPS points and orthomosaic GCPs for three dimensional correction of high resolution satellite images. They had also achieved to similar results from both surveyed GPS points and orthomosaic points. The geometric corrections by using GCPs surveying in land may give most accurate results [1]. Ke has made the experimental study of the process of orthorectification accuracy in the analysis of images VHRS. GCP properties (shapes, distribution, the accuracy, number of GCPs) were examined [6]. Yilmaz et al., have made their work available GCPs dimensions for IKONOS satellite imagery have researched [7]. Mutluoglu et al., in their study made woldview-2 satellite images have done a study on the appropriate size of GCPs [8]. _______________________________________________________________________________________________________________________________________________________________

WorldView-2 Satellite
The WorldView-2 (WV-2) satellite, launched by DigitalGlobe on Oct 8 2009 represents the first commercial imaging satellite to collect very high spatial resolution data in 8 spectral bands [9]. The images provided by the satellite can be used for applications such as mapping, land planning, disaster relief, exploration, defense and intelligence, visualization and simulation of environments, and classification. Worldview-2 can operate at an altitude of 770km with an inclination of 97.2° for a maximum orbital period of 100 minutes. WorldView-2's large-area collection capabilities and rapid retargeting are two important features of the satellite. WorldView-2's advanced geopositional technology provides significant improvements in accuracy. The accuracy specification has been tightened to 6.5m CE90 directly right off the satellite, meaning no processing, no elevation model and no ground control, and measured accuracy is expected to be approximately 4m CE90. WorldView-2 panchromatic resolution is 46cm and multispectral resolution is 1.8m. Distribution and use of imagery better than 0.50m GSD pan and 2.0m GSD multispectral is subject to prior approval by the U.S. Government. As the first high-resolution commercial satellite to provide eight spectral bands, WorldView-2 offers imagery with a high degree of detail, unlocking a finer level of analytical discernment that enables improved decisionmaking. In addition to industry-standard blue, green, red and near-infrared, WorldView-2 includes four previously unavailable bands, collected at 1.8m resolution: coastal blue, yellow, red edge and near-infrared 2. These bands offer a range of benefits to analysts, who will be able to identify a broader range of classification, (e.g. more varieties of vegetation or waterpenetrated objects), to extract more features (e.g. cotton-based camouflage from natural ground cover), to view a truer representation of colors that match natural human vision, and to track coastal changes and infractions. WorldView-2 data is distributed in five different levels, i.e., Basic 1B, Basic Stereo Pairs, Standard 2A, Ortho-Ready Standard (OR2A), and Orthorectified. OR2A has no topographic relief applied, making it suitable for custom orthorectification. OR2A is projected to an average elevation, either calculated from a terrain elevation model or supplied by the customer. OR2A products are recommended for geometric correction because the panchromatic and multispectral data are resampled to exactly the same geographic extents; hence, it is possible to perform pansharpening of the data before geometric correction if a pansharpened orthorectified image is desired [10], [11].

Test Area
A (15kmx14km) test area has created at surround of Konya Selcuk University Alaeddin Keykubad campus area. Application area of South and South East parts consist of the plains, Northern, North Eastern and North Western part consists of mountainous terrain. In the test area, outside the residential areas of land consist of agricultural areas and bare land. There are not height vegetation (height trees, forest). In the test area, the elevations are changing between 1000 m. and 1700 m. Frame area remain among the following geographic latitude and longitude and are given in table 1.

WorldView-2 Data
In this study, dated July 9, 2013, stereo woldview-2 (WV-2) satellite pansharpen images are used. With images (Rational Polynomial Coefficients) RPCs files are also obtained. RPC files containe orbital parameters of the satellites (orientation, height, etc.). RPC files have been provide model data of cameras to most software packages for 3D photogrammetric production of detail, digital elevation models and orthorectified imagery.

Office and Field studies
First of all; location of GCPs have been selected on the 1/25000 scaled map. Nonresidental places have been selected for GCP. The aim of this is prevent of GCP from the destroying and, secondly to use for other different studies. Steel Frame has been made for the GCPs. Selected GCPs have been set up using of steel frame poured concrete on the field. GCPs have been painted white and black according to measurements below (figure 2). In addition, GCPs are marked with calcareous at difficult to reach areas and certain details are selected at settled areas. GCPs coordinates were measured by the method of CORS-TR. Dual frequency Ashtech Promark 500 GNSS receiver was used at measurement. Sub 10 cm position accuracy with dualfrequency receiver can be obtained.    According to the adjustment results, when non-suitable control points have been selected, average errors at x and y direction have been not effected a lot but z values have been significantly changed at check points [14].

Conclusion
In this study 9 july 2013 dated 0.5 resolution, stereo WoldView-2 pansharpened images have been used. One of the widely using areas of VHRS is producing and updating of maps.
To use as a map of VHRS images, geometric correction should be done certainly. Rectification process can be done without GCPs, but, using GCPs are increasing the accuracy of rectification. Numbers, distribution, shape and spatial accuracy of GCPs have effect on the rectification accuracy. Time, cost and accuracy are an important factor at map production. If GCPs are marked more than needed, time and cost increase. This study has shown; rectification can be achieved at high accuracy with RPCs model and using suitable distribution of GCPs (4 or 5 GCPs). The effect of distribution of control points at the terrain have seen to be more effective than the effect of number of control points upon the adjustment results. Especially, z values are significantly changing at check points at rectification process when the non-suitable distributed GCPs have been used.