v.plant

NAME

v.plant - module to insert intermediate points on GRASS vector lines.
(GRASS Vector Program)

SYNOPSIS

v.plant
v.plant help
v.plant map=name

v.plant is an interactive program.

DESCRIPTION

This report describes the use and operation of the program v.plant, used with the GRASS GIS system. The report contains (i) a discussion of the problem which motivated the development of the program, (ii) a description of the method used, (iii) the manual page describing the usage of the program, (iv) a listing of the source-code.

Introduction

Map projections

Projections are representations of information defined on curved surfaces (usually spheroids) in two-dimensional space. This allows the description of the location information in cartesian or other plane coordinate systems. Particular projections are defined by precise mathematical relations, which allows conversion of maps from between projections to be achieved using straightforward mathematical operations [1].

Projecting line segments

When converted from one map projection to another, straight-lines become curves. Thus, a line segment which can be described by four parameters only (eg the coordinates of the end points) in one projection becomes an arc which needs more parameters in other projections. In GIS systems which describe arcs as a sequence of line segments, this means that intermediate points between the ends of the segment are used. The easiest method of projecting a line accurately is to include intermediate points in the description in the source projection, so that all these points are included in the target map.

Context within the GRASS GIS

Vector maps (lines and arcs) in the GRASS GIS are represented by a relatively straightforward format. This is exemplified in the ascii version of vector files, as produced by v.out.ascii [2] and described in the programmer's manual [3]. Individual vector segments are described by a sequence of coordinate pairs, with the number of coordinated pairs for the segment recorded in a brief header for each segment. The program v.prune [4] is provided to remove points which are considered redundant through being too close together within a segment. v.plant has been designed as the complement of this, to insert extra points at a specified spacing along straight-line segments.

Design of v.plant

v.plant has been implemented as a Bourne shell script. The basic procedure is as follows:

1. Write out the existing map in the dig_ascii format;

2. Use an included awk script to process each vector segment in turn, moving from point to point along the segment. If a span between two adjacent points is greater than the specified threshold (in map units) extra points are inserted collinearly.

3. The modified dig_ascii file is re-imported, overwriting the original file.

Because the original dig file is overwritten, all the support files are preserved. Since the vector segments remain in the same sequence, and all the original points remain in the modified map, including the end-points of each segment, all topology, attributes, etc are maintained.

The usage is exemplified in the attached manual page. The source code is also attached, which may be examined for a detailed understanding of the method.

TODO

A command-line version of v.plant would be highly desirable.

References

[1] Evenden, G.I. (1990) Cartographic projection procedures for the UNIX environment - a user's manual. USGS Open-File Report 90-284 (Also see Interim Report and 2nd Interim Report on Release 4, Evenden 1994).

[2] Higgins, M. & Westervelt, J. v.out.ascii - Converts a binary GRASS vector map layer into an ASCII GRASS vector map layer. GRASS 4.1 documentation (main section).

[3] Shapiro, M., Westervelt, J, Gerdes, D., Larson, M. & Brownfeld, K.R. (1993) GRASS 4.1 programmer's manual. US Army Construction Engineering Research Laboratory.

[4] Gerdes D., v.prune - Prunes points from binary GRASS vector data files. GRASS 4.1 documentation (main section).

CSIRO Exploration & Mining Report 239F, March 1996.

S.J.D. Cox