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PTGRID procedure

Generates a grid of points in a polygon (M.A. Mugglestone, S.A. Harding, B.Y.Y. Lee, P.J. Diggle & B.S. Rowlingson).

Option

PRINT = string token What to print (summary); default summ

Parameters

YPOLYGON = variates Vertical coordinates of each polygon; no default – this parameter must be set
XPOLYGON = variates Horizontal coordinates of each polygon; no default – this parameter must be set
NPOINTS = scalars How many points to generate
YSTEP = scalars Spacings to use between columns of the grid
XSTEP = scalars Spacings to use between rows of the grid
YGRID = variates Variates to receive the vertical coordinates of the points in the grid
XGRID = variates Variates to receive the horizontal coordinates of the points in the grid

Description

This procedure generates a grid of points in a polygon specified by the parameters XPOLYGON and YPOLYGON. The size of the grid may be specified in either of two ways. The first method is to specify the total number of points to be generated using the parameter NPOINTS. The value supplied for NPOINTS must be a positive integer. This method will produce a square grid, the number of rows and columns being approximately equal to SQRT(NPOINTS). The second method is to specify the required spacing between rows and columns of the grid using the parameters XSTEP and YSTEP. The values supplied for XSTEP and YSTEP should be on the scale of the coordinates of the polygon. If the parameter NPOINTS is set then any values specified for XSTEP and YSTEP will be ignored. The coordinates of the points which are generated may be saved using the parameters XGRID and YGRID.

Printed output is controlled by the PRINT option. The default setting of summary prints the horizontal and vertical coordinates of the points in the grid under the headings XGRID and YGRID.

Option: PRINT.

Parameters: YPOLYGON, XPOLYGON, NPOINTS, YSTEP, XSTEP, YGRID, XGRID.

Method

A procedure PTCHECKXY is called to check that XPOLYGON and YPOLYGON have identical restrictions. PTBOX is used to calculate the bounding box for the polygon specified by XPOLYGON and YPOLYGON. A grid of points spanning the bounding box is created according to the settings of NPOINTS (appropriately scaled to produce the equivalent density of points on the bounding box), XSTEP and YSTEP. Any points which fall outside the specified polygon are then removed using PTSINPOLYGON.

Action with RESTRICT

If XPOLYGON and YPOLYGON are restricted, only the subset of values specified by the restriction will be included in the calculations.

See also

Procedure: DPOLYGON.

Commands for: Spatial statistics.

Example

CAPTION 'PTGRID example'; STYLE=meta
VARIATE xpoly; VALUES=!(0,1,1,0)
&       ypoly; VALUES=!(0,0,1,1)
PTGRID  YPOLYGON=ypoly; XPOLYGON=xpoly; NPOINTS=16
Updated on March 6, 2019

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