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

X = Ax + By + C Y = Dx + Ey + F x 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 direction F = Translation in y direction k = sheer factor Skew angle = arctan(k) Sx = Scale factor in x direction Sy = 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- Order  Minimum of 6 control points Third - Order  Minimum of 10 control points First – Order is suitable for most purposes

Distortions  Tilt of the camera  Curvature of the earth  Uneven terrain Distortion can be corrected in the transformation and rectifying process.

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