Geographic Concepts

Geographic Concepts 3 fundamental concepts Datum Coordinate system Projections

Geographic Concepts Geodetic Datum: defines the position of the origin, scale, shape, and orientation of a 3-dimensional model of the earth.

Geographic Concepts Common Datums NAD27 (North American Datum): system derived from land-based surveys, using Clarke 1886 ellipsoid (Meades Ranch) NAD83: satellite-based system using the Earth’s center as a reference point; eventually adopted as GRS80 (Geodetic Reference System 1980) WGS84 (World Geodetic System 1984): mathematically refined GRS80 used by the US military and default for GPS (Doppler) For most uses, NAD83, GRS80, WGS84 are equivalent

Geographic Concepts Coordinate System: defines the “units of measure” of position with respect to the datum. Example: latitude and longitude in degrees, minutes, seconds or UTM coordinates in meters

Geographic Concepts

Geographic Concepts

Geographic Concepts Projection

What projections do Tissot’s Indicatrix of distortion - Eric Gaba

What projections do Tissot’s Indicatrix of distortion - Eric Gaba

Geographic Concepts Projections will compromise… Area Shape Scale Direction Select projection to fit your needs

Projection Geographic Mercator preserves NS distance preserves shape (terrible for poles, distorts area)

Projection Winkel Tripel (NGS solution) reduced distortion in both area and distance

Projection: Regional Squat California (Geographic) vs. Equal-Area California (Teale Albers)

Web Mercator Projection WGS84 Most web apps with Google Maps or ESRI basemaps (using their api) will be in Web Mercator, which is now the standard. In Web Mercator, like Mercator, preserves shape but distorts area and horrible at poles. Web Mercator Projection WGS84

Zoomed in detail shows the mid-lats in equal tiled sections which is convenient for most web browsing.