gxgy.to.index {CTFS} | R Documentation

## Converts GX GY Coordinates to a Quadrate Number

### Description

This function takes an x, y plot location and identifies the quadrate number. The quadrate is based on a gridsize that by default divides the plot into 20 by 20 m squares. The gridsize can be defined by the user so other quadrate sizes can be used. The inverse function is `index.to.gxgy`.

### Usage

    gxgy.to.index(gx, gy, gridsize = 20, plotdim = c(1000, 500))

### Arguments

`gx` | tree x coordinate
`gy` | tree y coordinate
`gridsize` | side of the square quadrate, 20 x 20m by default
`plotdim` | dimensions of the plot: east-west 1000m and north-south 500m

### Details

The function should work for any 4 sided plot, square or rectangular. The convention is 20 by 20 m quadrates, 1250 of them in a 1000 by 500 m plot. The convention is to number the quadrates starting in the southwest corner and going northward up a "column" of 25, and returning to the southern border for 26-50, etc. The columns are numbered 1 to 50, west to east (left to right). The rows are numbered 1 to 25, south to north (bottom to top).

### Value

Returns a vector, the index number of the quadrate (default values range from 1 to 1250). Returns NA when an invalid gx or gy is provided.

### Author(s)

Rick Condit, Suzanne Lao and Pamela Hall

### See Also

`CTFS.quadfunctions`

### Examples

    ## Not run:

    # the corners:
    tst.gx=c(0,0,999,999)
    tst.gy=c(0,499,0,499)
    gxgy.to.index(tst.gx,tst.gy)

    # the far edge from 0,0 is out of bounds
    gxgy.to.index(500,1000)

    # any location in the conventional plot
    gxgy.to.index(321,123)

    # same location in smaller plot yields same index
    gxgy.to.index(321,123,plotdim=c(500,500))

    # index for hectare sized quadrates
    gxgy.to.index(321,123,gridsize=100)
    ## End(Not run)

[Package CTFS version 1.00 Index]