GEOGRAPHIC ANALYSIS MODULES

Database Query

RECLASS

Reclassifies pixels by equal intervals or user-defined schemes. RECLASS is commonly used as a database query routine by reclassifying images into Boolean maps of areas meeting the specified conditions.

OVERLAY

Undertakes pixel-wise addition, subtraction, multiplication, division and exponentiation of paired images. Maximum, minimum, -normalized ratio+ and -cover+ are also supported. On binary images, logical AND and OR operations are supported by means of the multiply and maximum operation. Other boolean operations are supported through various binary image overlay combinations.

PCLASS

Evaluates the probability that pixels exceed or are exceeded by a specified threshold using the stated RMS error or the data set and a normal probability distribution error model.

QUERY

Extracts pixels designated by an independent mask into a sequential file for subsequent statistical analysis.

AREA

Creates a new image by giving each output pixel the value of the area of the class to which the input pixel belonged. Output can also be produced as a table or an attribute values file in a range of measurement units.

PERIM

Creates a new image by giving each output pixel the value of the perimeter of the class to which the input pixel belongs. Output can also be produced as a table or an attribute values file in a range of measurement units.

see also ASSIGN (as an optional reclassification routine), EXTRACT , HISTO and CROSSTAB .

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Map Algebra

SCALAR

Adds, subtracts, multiplies, divides and exponentiates pixels by a constant value.

TRANSFOR

Transforms the attributes of images such as log transformation, using any of 14 operations.

see also OVERLAY .

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Distance Operators

DISTANCE

Calculates the distance/proximity of each pixel to the nearest of a set of target pixels.

COST

Generates a distance/proximity surface where distance is measured as the least cost distance in moving over a friction surface.

VARCOST

Computes an anisotropic cost surface. This module is the equivalent of COST (specifically the COSTPUSH algorithm) except that it recognizes that frictions may have different strengths in different directions. Frictions are entered as force vectors described by a pair of images - a magnitude image and a direction image. VARCOST also allows the specification of an isotropic friction image for omni-directional elements.

DISPERSE

DISPERSE is a companion to VARCOST that calculates dispersion distances. The procedure is essentially the same as that of VARCOST except that the emphasis is on forces acting to disperse a body (e. g., a point-source contaminant spill) and the time taken to disperse it to a specified distance. DISPERSE is released as an experimental module in Version 4.1 and all users are invited to comment upon their experiences with this procedure and provide suggestions for modifications.

RESULTAN

Computes the resultant force vector (as a magnitude and direction image pair) from two input force vector image pairs.

DECOMP

Decomposes a force vector (as a magnitude and direction image pair) into X and Y component images. Also takes X and Y component images and produces a force vector image pair.

SPDIST

A companion to the DISTANCE module, SPDIST calculates spherical distances on the surface of the earth from designated features using spherical trigonometry.

PATHWAY

Finds the shortest path between one or more specified points and a destination specified as the lowest point on a cost surface.

ALLOCATE

Performs spatial allocation using either DISTANCE or COST surfaces.

THIESSEN

Produces Thiessen polygons (a Voronoi Tesselation) about a set of irregularly distributed points.

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Context Operators

SURFACE

Produces slope gradient and aspect images from a surface. Can also be used to create analytical hillshading.

GROUP

Classifies pixels according to contiguous groups.

VIEWSHED

Creates an image of all points visible from one or more viewpoints over a given surface.

WATRSHED

Determines the watersheds of one or more specified locations.

HNTRLAND

Determines the supply areas dominated by, or the hinterlands of, point demand centers.

see also FILTER .

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