Post on 07-Feb-2016
description
Digitization
Source: NAMRIA
Digitization Procedure
� Display the georeferenced image
� Set the map units to meters
� Create a new theme/layer
� Zoom to the image until pixels are big enough to be seen
� Set the appropriate snap tolerance to automatically connect two lines
Digitization Procedure
� Start digitizing. Digitize in the middle or center of the feature. A tolerance limit of equal to and less than
0.25 mm (equal to 12.5m error) away from the midpoint is acceptable.
� Make an attribute table corresponding to the features created.
� Edit feature, if necessary.
4.235 m. (0.0847 mm)
Digitization
4.235 m.
(ground)
4.235 m.
(ground)
4.235 m.
(ground)
Projections, Georeferencing and Geometric Rectification
Source: NAMRIA
Digitization
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Georeferencing/Geometric
Rectification� Georeferencing is establishing relationship
between an image coordinate system (row, column) and a map coordinate system (x, y or longitude, latitude or Easting, Northing). It is also the process of assigning map coordinates to image data.
� Geometric rectification is the process by which image or grid is converted from image coordinates to real-world coordinates.Ittypically involves rotation and scaling of grid cells, and thus requires resampling of values.
Georeferencing/Geometric
Rectification� The pixels of the new grid will probably not align
perfectly with the pixels of the original grid, so
resampling is needed.
� Resampling is the process of extrapolating data
values for pixels on the new grid from the values
of the source pixels.
� Rectification, by definition, involves
georeferencing, since all map projection systems
are associated with map coordinates.
Procedure
� Rectification is accomplished by
– Matching points of known location in both image and map or image coordinate system
– These points are used to compute the best fit polynomial transformation.
At least, the 4 corners should be
selected as control points for georeferencing.
10’ x 15’ Map
In selecting more points in additionto the 4 corner ticks, make sure that thereference pointsare well-distributed.
Selecting / Locating Points
� Intersection of roads and other man-made features that can to located in both map and image.
� GPS points of ground locations that can be identified in the image.
� Points should be located evenly across the image (sometimes difficult in natural areas)
Transformation Models
Transformation Models Minimum No. of Points
Affine Transformation 4 points
Second Degree Polynomial 6 points
Affine Transformation
Affine Transformation
� A linear correction model that involves three parameters for X and Y.
� The three parameter represent shifts in
– X or Y direction
– Scale
– Rotation about a point.
Root Mean Square� RMS is the distance between the input (source)
location of a control point and the retransformed location for the same control point.
� The smaller the RMS the better. It is the measure of best fit to the actual locations of map coordinates.
� RMS error is normally expressed as a distance in pixel width.
– If 1 pixel for a digital 1:50,000 topographic map scanned at 300 dpi is equivalent to 4.325 m. on the ground, then an RMS error of 2 will be equal to 2 x 4.325 m = 8.65 m. on the ground.
Root Mean Square
Root Mean Square
Root Mean Square
Root Mean Square
Root Mean Square