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