Lec_6_Intro to geo-referencing
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This document discusses geo-referencing raster data. It defines geo-referencing as aligning raster data to real-world coordinates so it can be viewed and analyzed with other geographic data. There are two main types of geo-referencing: absolute, which aligns raster to maps or coordinates, and relative, which aligns raster to other geo-referenced raster. The document outlines the geo-referencing process, including selecting ground control points, performing transformations, and interpreting error metrics to evaluate accuracy.
Lec_6_Intro to geo-referencing
- 1. Introduction to Geo- Referencing Prof. Dr. Sajid Rashid Ahmad sajidpu@yahoo.com Atiqa Ijaz Khan _ Demonstrator atiqa_ss09@yahoo.com
- 2. Basics of Raster Data • Raster data is commonly obtained by scanning maps or collecting aerial photographs and satellite images. • Scanned map datasets don't normally contain spatial reference information (either embedded in the file or as a separate file). • With aerial photography and satellite imagery, sometimes the location information delivered with them is inadequate, and the data does not align properly with other data you have. • Thus, to use some raster datasets in conjunction with your other spatial data, you may need to align or georeference them to a map coordinate system. • A map coordinate system is defined using a map projection (a method by which the curved surface of the earth is portrayed on a flat surface). Tuesday, December 23, 2014Institute of Geology, University of the Punjab 2
- 3. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 3 Upper Left: Topographic Sheet of India and Pakistan Upper Right: Satellite Image Lower Right: Aerial Photography
- 4. Geo-Referencing • “Georeferencing is the process of aligning the raster dataset, with the help of real world coordinates, to its actual position on the earth (globe)” (ESRI). • Georeferencing is refers to the process of assigning map coordinates to image data. • Georeferencing means to define its existence in the physical space. • “Rectification is also known as Geo-Referencing” (Swan, 1988). • “The alignment of an image to a map so that the image is planimetric , just like the map” (Jensen, 2004). • “The process by which the geometry of an image is made planimetric” (Jensen, 1986) Tuesday, December 23, 2014Institute of Geology, University of the Punjab 4
- 5. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 5 Geo-Referenced Map of Pakistan
- 6. Why Geo-Referencing? • When data from different source is to be combined in GIS analysis, it is required to align them properly with the help of geo-referencing. • As the raster is made up of pixels, it does not store any information regarding location, so we cant use it into real world location until it is geo-referenced. • To calculate the exact area or extent of a raster dataset. • Satellite images inherent some un-systematic errors making its location shifted to the original xy-location. • Georeferencing is the process of aligning the geographic dataset, to a known coordinate system so it can be viewed, queried, and analysed with other geographic data. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 6
- 7. Types of Geo-Referencing • Geo-Referencing is divided into two further types, like: • Absolute Geo-Referencing • Relative Geo-Referencing • Absolute Geo-Referencing(Image to Map Rectification): • The raster image is geo-referenced using: • Google Earth • Topo-Sheets • Ground Control Points (GCPs) • Relative Geo-Referencing (Image to Image Registration): • The raster image is geo-referenced using another already referenced image. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 7
- 8. Image to Image Rectification • It is a process in which geometry of an image is made planimetric. • It is used in order to accurately measure the area, direction and distances. • This process mainly involves: Selecting GCPs from image to map with its counter- part available on the map. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 8
- 9. Image to Image Registration • It is a process in which two images of like geometry and same geographical location are positioned in a way that it coincides with each other. • It involves translational and rotational alignment. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 9
- 10. Aligning Raster using Control Points • Control Points (CPs): • Could be defined as a point on the surface of the earth of known location (i.e. fixed within an established co-ordinate system) which is used to geo-reference image data sources, such as remotely sensed images or scanned maps. • It will help in shifting the raster datasets from its original location to spatially corrected location. • One control point from a raster dataset associates with one and only CP from the target map that is a geo-referenced raster or vector data. • The connection between these points is said to be a ‘Link’. • Complexity of the image defines how many number of links are to be create. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 10
- 11. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 11 CPs Control Points
- 12. • It is not necessary, the large number of links always produces better registration. • Typically, try to link one corner point and few at the interior to have best results. • Generally, greater the overlap between the raster and target data, the more the data is accurately registered. • It is not a good practice to establish control point only at one edge of an image. • It should be well distributed overall the image. • The quality of image registration is only up-to the limit to which is it registered. • To get the good results, one must geo-referenced the image to the highest possible resolution available. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 12
- 13. Raster Transformation • Geo-referencing involves transformation of raster so to establish a strong control. • It involves different techniques: • Shift (Zero Order) • Affine (1st Order) • 2nd Order • 3rd Order • Spline • Adjust • Project Transformation Tuesday, December 23, 2014Institute of Geology, University of the Punjab 13
- 14. Zero Order (Shift) • A zero-order polynomial is used to shift your data. • This is commonly used when your data is already georeferenced, but a small shift will better line up your data. • Only one link is required to perform a zero-order polynomial shift. • It may be a good idea to create a few links, then choose the one that looks the most accurate. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 14
- 15. 1st Order (Affine) • The first-order polynomial transformation is commonly used to georeference an image. • Use a first-order or affine transformation to shift, scale, and rotate a raster dataset. • This generally results in straight lines on the raster dataset mapped as straight lines in the warped raster dataset. • Thus, squares and rectangles on the raster dataset are commonly changed into parallelograms of arbitrary scaling and angle orientation. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 15
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- 17. Polynomial Order • With a minimum of three links, the mathematical equation used with a first- order transformation can exactly map each raster point to the target location. • Any more than three links introduces errors, or residuals, that are distributed throughout all the links. • However, you should add more than three links, because if one link is positionally wrong, it has a much greater impact on the transformation. • Thus, even though the mathematical transformation error may increase as you create more links, the overall accuracy of the transformation will increase as well Tuesday, December 23, 2014Institute of Geology, University of the Punjab 17
- 18. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 18 The number of the non correlated control points required for this method must be: • 1 for a zero-order shift, • 3 for a first order affine, • 6 for a second order, and • 10 for a third order. The lower order polynomials tend to give a random type error, while the higher order polynomials tend to give an extrapolation error.
- 19. High Order Transformation • The higher the transformation order, the more complex the distortion that can be corrected. • However, transformations higher than third order are rarely needed. • Higher-order transformations require more links and, thus, will involve progressively more processing time. • In general, if your raster dataset needs to be stretched, scaled, and rotated, use a first-order transformation. • If, however, the raster dataset must be bent or curved, use a second- or third- order transformation. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 19
- 20. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 20
- 21. Spline • The spline transformation is a true rubber sheeting method and optimizes for local accuracy but not global accuracy. • Spline transforms the source control points exactly to target control points. • The pixels that are a distance from the control points are not guaranteed to be accurate. • This transformation is useful when the control points are important, and it is required that they be registered precisely. • Adding more control points can increase overall accuracy of the spline transformation. • Spline requires a minimum of 10 control points. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 21
- 22. Interpreting the Root Mean Square • When the general formula is derived and applied to the control point, a measure of the error — the residual error — is returned. • Residual Error: The error is the difference between where the from point ended up as opposed to the actual location that was specified—the to point position. • The total error is computed by taking the root mean square (RMS) sum of all the residuals to compute the RMS error. • This value describes how consistent the transformation is between the different control points (links). • When the error is particularly large, you can remove and add control points to adjust the error. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 22
- 23. RMS Error • Although the RMS error is a good assessment of the transformation's accuracy, don't confuse a low RMS error with an accurate registration. • As the larger the number of control points, the more accurately polynomial transforms, and give low RMS value. • Typically, the adjust and spline transformations give an RMS of nearly zero or zero; however, this does not mean that the image will be perfectly georeferenced. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 23
- 24. Residual • All residuals closer to zero are considered more accurate. • The other types of residual are: • Forward: • The forward residual shows you the error in the same units as the data frame spatial reference. • Inverse: • The inverse residual shows you the error in the pixels units. • Forward-Inverse: • The forward-inverse residual is a measure of how close your accuracy is, measured in pixels. Tuesday, December 23, 2014Institute of Geology, University of the Punjab 24
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