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Georeferencing
Getting maps and satellite images into GIS
Georeferencing (ESRI PRESS 2003)
To establish a relationship between page coordinates on a planar map and known real-world coordinates.
Geometric Transformation (Chang)The process of converting a map or an
image from one coordinate system to another by using a set of control points and a transformation equation.
Geometric transformation and georeferencing often involves: scaling, rotating, and warping an image to a giving set of geographic or projected coordinates
Sources of Raster Imagery
Scanned Images Historic Paper Maps Aerial photographs
Digital Aerial Images
Satellite Images
Control Points
Control points are know locations for a physical feature that can be identified.
These are the points you will be using to georeference your image.
Control points can be collected using GPS, determined using tics on a paper map, or from known features from a base layer
Transformation Methods
different methods preserve different geometric properties
Most common!
First-Order Transformation - Affine
First-Order Transformation - Affine
X = Ax + By + C Y = Dx + Ey + Fx and y are input coordinates
X and Y output coordinates - to be determined
A = Sxcos(t)B = Sy[k cos(t) –sin(t)]D = Sxsin(t)E = Sy[k sin(t) + cos(t)]C = Translation in x directionF = Translation in y direction k = sheer factor Skew angle = arctan(k)Sx = Scale factor in x directionSy = Scale factor in y direction
•Scales, skews, rotates, and translates the layer coordinates.•The affine transformation requires a minimum of three control points.
Higher Order Transformation
Complex distortions can be corrected
Requires more links and control points
Second- OrderMinimum of 6 control points
Third - Order Minimum of 10 control points
First – Order is suitable for most purposes
Distortions
Distortions
Tilt of the cameraCurvature of the earthUneven terrain
Distortion can be corrected in the transformation and rectifying process.
Resampling
Resampling A process of filling each pixel of a newly
transformed image with a value or a derived value from the original value.
Resampling Methods Nearest neighborhood assignment Bilinear interpolation (four neighbors) Cubic convolution (16 neighbors)
Pyramiding Technique that builds different levels of
resolution of data for display (works behind the scene in ArcGIS)
Resampling
Root Mean Square (RMS) Error
Deviation between the actual location and the estimated location of the control points.
Error for a control point is
Average RMS is
General Steps for Georeferencing an Image in ArcGIS
1. Obtain a digital or scanned imagery/map .
2. Obtain base data (a data layer with a known coordinate system) or control points that represent locations/objects visible in image.
3. Create displacement links (links), clicking first on RASTER, then on base layer.
4. Look at the link table for acceptable residual on each point and total RMS error.
5. Select transformation method.
6. Rectify the map by selecting a resampling method (optional in ArcMap). Rectifying will create a new image file