Documentation
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Overview ¶
Package projections provides map projections for converting between WGS84 geographic coordinates and flat projected coordinate systems.
Index ¶
Constants ¶
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Variables ¶
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Functions ¶
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Types ¶
type Option ¶
type Option func(*TransverseMercator)
Option configures a TransverseMercator projection.
func WithCenter ¶
WithCenter sets the central meridian and origin latitude from a geographic point (orb.Point{longitude, latitude} in degrees). This is a convenience option that sets both values at once.
func WithCentralMeridian ¶
WithCentralMeridian sets the longitude (in degrees) of the central meridian.
func WithFalseEasting ¶
WithFalseEasting sets the constant offset (in meters) added to easting coordinates so that all values in the useful area are positive.
func WithFalseNorthing ¶
WithFalseNorthing sets the constant offset (in meters) added to northing coordinates so that all values in the useful area are positive.
func WithOriginLatitude ¶
WithOriginLatitude sets the latitude (in degrees) used as the northing origin.
func WithScaleFactor ¶
WithScaleFactor sets the scale factor at the central meridian. For UTM and DCS World projections this is typically 0.9996.
type Projection ¶
type Projection interface {
// ToProjected converts a WGS84 point (orb.Point{longitude, latitude} in
// degrees) to projected coordinates (orb.Point{easting, northing} in
// meters).
ToProjected(point orb.Point) orb.Point
// ToWGS84 converts projected coordinates (orb.Point{easting, northing} in
// meters) back to WGS84 geographic coordinates (orb.Point{longitude,
// latitude} in degrees).
ToWGS84(projected orb.Point) orb.Point
}
Projection converts between WGS84 geographic coordinates and a flat projected coordinate system.
type TransverseMercator ¶
type TransverseMercator struct {
// contains filtered or unexported fields
}
TransverseMercator converts between WGS84 geographic coordinates (longitude/latitude in degrees) and a flat Transverse Mercator coordinate system (easting/northing in meters).
A Transverse Mercator projection works by conceptually wrapping a cylinder around the Earth so that it touches along a chosen line of longitude (the central meridian) instead of along the equator. Points on the ellipsoid are then projected onto this cylinder and unrolled into a flat surface. Near the central meridian the distortion is very small, making it a good local coordinate system for distance and angle calculations. Farther from the central meridian, distortion grows — which is why each DCS terrain uses its own projection centered on that terrain's longitude.
func NewTransverseMercator ¶
func NewTransverseMercator(opts ...Option) *TransverseMercator
NewTransverseMercator creates a Transverse Mercator projection with the given options. The default scale factor is 1.0; all other parameters default to 0.
func (*TransverseMercator) ToProjected ¶
func (p *TransverseMercator) ToProjected(point orb.Point) orb.Point
ToProjected converts a WGS84 point (orb.Point{longitude, latitude} in degrees) to flat projected coordinates (orb.Point{easting, northing} in meters).
The implementation uses the series expansion from "Map Projections: A Working Manual" (https://pubs.usgs.gov/pp/1395/report.pdf), Chapter 8: Transverse Mercator projection.
func (*TransverseMercator) ToWGS84 ¶
func (p *TransverseMercator) ToWGS84(projected orb.Point) orb.Point
ToWGS84 converts projected coordinates (orb.Point{easting, northing} in meters) back to WGS84 geographic coordinates (orb.Point{longitude, latitude} in degrees).
The inverse projection is not a simple algebraic inversion of the forward formulas. Instead it uses the "footpoint latitude" approach (Snyder eq. 8-18 through 8-23): first, the northing is converted back to a meridional arc distance, then a series expansion recovers an approximate latitude (the footpoint), and finally correction terms refine the latitude and recover the longitude.
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