CRTHMR

Lines to Thematic Raster

Description

This module builds a thematic raster (THMR) from lines (and optional centroids). The input lines that make-up the closed shapes can come from multiple layers. The resultant thematic raster has a system procuced attribute to report the correctness of results.

Ports

Port details

VectorInputLayer: Vector Input Layer

Contains the input lines to create the thematic raster. The input lines may come from multiple layers. e.g. the lines to form the closed shapes may have a map boundary on one layer, soil type lines on a second layer and water boundary lines on a third layer.

DBInputLayer: Centroid Input Layer

Contains the input attribute layer. Equivalent to an input centroid layer that may already contain "user attributes"

InputArea: Bounding Area

Contains the input bounding area.

Output: Thematic Raster Output Layer

Contains the output thematic raster.

Parameters

Parameter details

REDUCTIONFACT: Reduction Factor

Valid Values: 1,3,5,7,9 Default value: 1

The Reduction Factor is a divisor to be applied to the X and Y pixel size for processing purposes. It allows the input lines to be processed at a finer resolution for more accusrate results. This reduced pixel size is only temporary, it will be aggregated back to the specified X and Y pixel size for final storage and hence display. e.g. X and Y pixel size = 20m, reduction factor = 5, since the reduction factor is a divisor the processing pixel size = 4 (20/5).

CALCPERIMETER: Calculate Perimeter?

Valid Values: TRUE,FALSE Default value: FALSE

If checked the perimeter is calculated and stored in an attribute table geometry field called "Perimeter". Area is automatically calculated.

MINPOLYPIXLE: Minimum number of pixels per polygon

Valid Values: 1 < x <= 100 Default value: 1

Specifies the minimum number of pixels per polygon. Avoids creating many small, insignificant thematic raster areas. It can be useful for removing potential thematic raster areas caused by slivers. Resultant thematic raster areas that have less pixels than this "Minimum Number" get merged into their neighbor.

XPixSize: Pixel X Size

Valid Values: x > 0 Default value: 30.0

Specifies the X pixel size in file units of the output thematic raster.

YPixSize: Pixel Y Size

Valid Values: y > 0.0 Default value: 30.0

Specifies the Y pixel size in file units of the output thematic raster.

A note on the Pixel X size and Pixel Y size: these parameter fields become disabled when you use an "InputArea" on the Files tab | Input Ports. This InputArea sets a bounding area which defines the extents, projection and pixel size of the output thematic raster

Details

A thematic raster (THMR) is the raster equivalent of a topological polygon layer. The THMR is different from a "raster"(e.g. image channel or DEM) in that the pixel value is linked to an attribute table record via its pixel value and a corresponding attribute table "Join Field" value. The default created "Join Field" is an attribute called "PixelValue"

Create Thematic Raster lays pixels of user-defined size over the input line work, determines which closed shape the pixel belongs to, links all the pixels in each closed shape to that new thematic raster areas attribute record and finally reports on the correctness of the newly created thematic raster.

The input linework, to form the closed shapes, can come from multiple layers but the linework should be of the same projection and datum. Line work required to form a thematic raster doesen't necesssarily have to be "clean linework", CRTHMR ignores gaps in lines if the gap length is smaller than the specified pixel size. For example, if you specify a pixel size of 20m (reduction factor=1) gaps less than 20m will be ignored, while gaps greater than 20m will be reported as an error. The DBInput Layer is optional, it is a point layer that acts as centroids for the closed shape. If the user specifies an input centroid layer a copy is made to be used as the database to be "joined" to the thmenatic raster. If no layer is specified the system creates a new one, with only the default sytem fields required by this function.

The perimeter is optional because it is computationally expensive, it sums the diagonal distances of the pixels along the boundary of the input line work closed shape.

If the program finds a "same value" group of contiguous pixels which is less than the specified number in the "Minimum pixels per polygon"these pixels are merged into the thematic raster area on the left. If no left area exists the pixels merge right.

Pixel Size: Smaller pixel sizes conform more closely to the input linework forming the closed shapes, which generally produces a more accurate area and perimeter calculation, it also produces a smoother edge in the display and helps reduce the possibility of "pinch-offs". The negatives of a smaller pixel size are increased file size and slower processing times. Pinch-offs occur where two lines (for example, two lines defining river banks) come closer together then the size of a single pixel. The program would then see this as a closed shape. To avoid pinch-offs, the pixel size chosen should be no more then two-thirds of the minimum gap considered to be "open", e.g. smallest distance between the river banks is 15m, then set pixel to no more than 10m. The Reduction Factor is mainly to allow a smaller processing pixel size in order to avoid pinch-offs.

Note: if the target output file is an existing file and its "Master Georeferencing Segment" pixel size is different to your "InputParams" set pixel size then the output will be resampled to fit and hence may no longer match your input linework. If you wish to keep the thematic raster in the same file as the source lines then make sure that files "Master Georeferencing Section's" set pixel size is equal to the final pixel size you require and its extents are at least equal to the extents of the input linework.

The "correctness" results are stored in the output thematic rasters attribute table in a system attribute called "Errortheme". The results can be seen by applying a "representation" based on "Errortheme, hence the display will show a color based on the "errortheme" numeric value. A correct thematic raster has one attribute record per thematic raster area (closed shape), therefore if the user inputs a DBInputLayer the program is checking for one centroid per closed shape. More than one or zero centroids will report as an error via the "errortheme" attribute. The program default is to automatically create missing attribute records.

"Errortheme" gets assigned a number based on the findings of the centroid to thematic raster area correspondance.The "errortheme" values have the following meanings:

3 = Thematic raster area had a missing centroid (this could also be the result of a pinch-off) and the program placed one (This is the case when no DBInputLayer is specified, therefore all records will have "errortheme" =3 and hence when the representation is aplied based on "errortheme" all thematic raster areas will all be displayed in the color equal to color number 3)

4 = Correct closed shape to centroid correspondence. If you had a DBInputLayer specified then if all was correct all "errortheme" values would be 4, all pixels in the thematic raster where correctly assigned to a record.

5 = Invalid linkage, which is the centroid is out of the linework extents.

6 = The centroid was linked to this record in cases of duplicate centroids in the closed shape. This will always have at least one corresponding record that has an "errortheme" of 7.

7 = The centroid was not linked to this record as it was a duplicate in the closed shape, hence no pixels in the thematic raster where assigned to the is record======= Specifies the y pixel size for the resulting thematic raster. This parameter is not required and will be disabled if the georeferencing is specified from the input port.