R: Add Tissot Indicatrices to a Map

mapTissot {oce} | R Documentation

## Add Tissot Indicatrices to a Map

### Description

Plot ellipses at grid intersection points, as a method for indicating the distortion inherent in the projection, somewhat analogous to the scheme used in reference 1. (Each ellipse is drawn with 64 segments.)

### Usage

    mapTissot(grid = rep(15, 2), scale = 0.2, crosshairs = FALSE, ...)

### Arguments

`grid` | numeric vector of length 2, specifying the increment in longitude and latitude for the grid. Indicatrices are drawn at e.g. longitudes `seq(-180, 180, grid[1])`.
`scale` | numerical scale factor for ellipses. This is multiplied by `min(grid)` and the result is the radius of the circle on the earth, in latitude degrees.
`crosshairs` | logical value indicating whether to draw constant-latitude and constant-longitude crosshairs within the ellipses. (These are drawn with 10 line segments each.) This can be helpful in cases where it is not desired to use `mapGrid()` to draw the longitude/latitude grid.
`...` | extra arguments passed to plotting functions, e.g. `col="red"` yields red indicatrices.

### Author(s)

Dan Kelley

### References

1. Snyder, John P., 1987. Map Projections: A Working Manual. USGS Professional Paper: 1395

### See Also

A map must first have been created with `mapPlot()`.

Other functions related to maps: `formatPosition()`, `lonlat2map()`, `lonlat2utm()`, `map2lonlat()`, `mapArrows()`, `mapAxis()`, `mapContour()`, `mapCoordinateSystem()`, `mapDirectionField()`, `mapGrid()`, `mapImage()`, `mapLines()`, `mapLocator()`, `mapLongitudeLatitudeXY()`, `mapPlot()`, `mapPoints()`, `mapPolygon()`, `mapScalebar()`, `mapText()`, `oceCRS()`, `shiftLongitude()`, `usrLonLat()`, `utm2lonlat()`

### Examples

    library(oce)
    data(coastlineWorld)
    par(mfrow=c(1, 1), mar=c(2, 2, 1, 1))
    p  <- "+proj=aea +lat_1=10 +lat_2=60 +lon_0=-45"
    mapPlot(coastlineWorld, projection=p, col="gray",
        longitudelim=c(-90,0), latitudelim=c(0, 50))
    mapTissot(c(15, 15), col="red")

[Package oce version 1.7-2 Index]