Georeferencing a Sketch

1
Georeferencing a Sketch

• GPS for GIS
• Jerry Davis

2
Georeferencing

• Refers to tying dataset coordinates to a known,
  real-world coordinate system
• This allows
• Geographically coincident data to overlay for
  analytical or cartographic purposes
• Geographically adjacent data to be merged for
  analytical or cartographic purposes
• Accurate direction, distance, and area
  measurements

3
Transformation

• Converting from one coordinate space to another
  by
• Shifting origin
• Rotating
• Scaling coordinates
• Skewing
• With higher-order, can change further

4
Transformation Order

• First Order (affine) three are needed
• First point translates
• Second point rotates and rescales
• Third point defines a plane, so distorts in 2
  dimensions
• Second and Third Order six or ten links needed
• Allows map to curve to fit.

5
Transformation Procedure

• Identify at least 4 control points
• Locations whose real world coordinates are known
  or can be determined
• Create links
• The from (source) and to (destination)
  coordinates of these 4 points

• Determine fit
• Remember, theres no stretching to make all
  points fit, only shift, scale, rotate, skew
• Reported as RMS (root mean square) error
• The average error of the 4 points
• Reported in source and destination units
• Determine if amount of error is acceptable for
  your use of data

6
Finding Control Points

• Source map may have tics labeled with location
• Lat long?
• These can be converted to the source maps
  projection/coordinate system using GIS tools
• e.g. change Lat long to UTM

• You can find features that are also on other
  geographically controlled maps or digital data
• e.g. stream or road intersection
• Acquire source coordinates by georeferencing the
  other paper map
• Or directly from other georeferenced digital
  data
• Or GPS location in field

7
Ungeoreferenced Sketch
8
With two three points rotation scaling

• Links created between
• 201 on sketch point
• 215 on sketch point
• Enough info for
• Translation
• Rotation
• Rescaling
• With 3 points
• Can rescale differently in X Y

9
Four points, residual error, RMSE

• With four points
• Each point will be off a bit
• Residual error in Link Table shows how much
• Least Squares solution
• Total is RMSE

10
Six Points Affine
11
Fifteen Point Affine