Lec_6_Intro to geo-referencing

Introduction to Geo-Referencing

Prof. Dr. Sajid Rashid Ahmad

[email protected]

Atiqa Ijaz Khan _ Demonstrator

[email protected]

Basics of Raster Data

• Raster data is commonly obtained by scanning maps or collecting aerialphotographs and satellite images.

• Scanned map datasets don't normally contain spatial reference information (eitherembedded in the file or as a separate file).

• With aerial photography and satellite imagery, sometimes the location informationdelivered with them is inadequate, and the data does not align properly with other datayou 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 thecurved surface of the earth is portrayed on a flat surface).

Tuesday, December 23, 2014Institute of Geology, University of the Punjab

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Upper Left:

Topographic Sheet of India

and Pakistan

Upper Right: Satellite

Image

Lower Right:

Aerial Photography

Geo-Referencing

• “Georeferencing is the process of aligning the raster dataset, with the help ofreal world coordinates, to its actual position on the earth (globe)” (ESRI).

• Georeferencing is refers to the process of assigning map coordinates to imagedata.

• 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 themap” (Jensen, 2004).

• “The process by which the geometry of an image is made planimetric” (Jensen,1986)


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Geo-Referenced Map of Pakistan

Why Geo-Referencing?

• When data from different source is to be combined in GIS analysis, it is requiredto align them properly with the help of geo-referencing.

• As the raster is made up of pixels, it does not store any information regardinglocation, 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 tothe original xy-location.

• Georeferencing is the process of aligning the geographic dataset, to a knowncoordinate system so it can be viewed, queried, and analysed with othergeographic data.


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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.


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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.


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Image to Image Registration

• It is a process in which two images of like geometry and same geographicallocation are positioned in a way that it coincides with each other.

• It involves translational and rotational alignment.


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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-referenceimage data sources, such as remotely sensed images or scanned maps.

• It will help in shifting the raster datasets from its original location to spatiallycorrected location.

• One control point from a raster dataset associates with one and only CP from thetarget 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.


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CPsControl

Points

• 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 thedata 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 highestpossible resolution available.


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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


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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 shiftwill 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 themost accurate.


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1st Order (Affine)

• The first-order polynomial transformation is commonly used to georeference animage.

• Use a first-order or affine transformation to shift, scale, and rotate a rasterdataset.

• This generally results in straight lines on the raster dataset mapped as straightlines in the warped raster dataset.

• Thus, squares and rectangles on the raster dataset are commonly changed intoparallelograms of arbitrary scaling and angle orientation.


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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 targetlocation.

• Any more than three links introduces errors, or residuals, that are distributedthroughout all the links.

• However, you should add more than three links, because if one link is positionallywrong, it has a much greater impact on the transformation.

• Thus, even though the mathematical transformation error may increase as youcreate more links, the overall accuracy of the transformation will increase as well


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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.

High Order Transformation

• The higher the transformation order, the more complex the distortion that can becorrected.

• However, transformations higher than third order are rarely needed.

• Higher-order transformations require more links and, thus, will involveprogressively more processing time.

• In general, if your raster dataset needs to be stretched, scaled, and rotated, use afirst-order transformation.

• If, however, the raster dataset must be bent or curved, use a second- or third-order transformation.


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Spline

• The spline transformation is a true rubber sheeting method and optimizes forlocal 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 beaccurate.

• This transformation is useful when the control points are important, and it isrequired that they be registered precisely.

• Adding more control points can increase overall accuracy of the splinetransformation.

• Spline requires a minimum of 10 control points.


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Interpreting the Root Mean Square

• When the general formula is derived and applied to the control point, a measureof the error — the residual error — is returned.

• Residual Error: The error is the difference between where the from point endedup 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 theresiduals to compute the RMS error.

• This value describes how consistent the transformation is between the differentcontrol points (links).

• When the error is particularly large, you can remove and add control points toadjust the error.


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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 polynomialtransforms, and give low RMS value.

• Typically, the adjust and spline transformations give an RMS of nearly zero orzero; however, this does not mean that the image will be perfectly georeferenced.


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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 spatialreference.

• 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 inpixels.


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Lec_6_Intro to geo-referencing

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