1. Geographic Concepts

2. Geographic Concepts 3 fundamental concepts Datum Coordinate system
Projections

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

4. 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

5. 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

6. Geographic Concepts

7. Geographic Concepts

8. Geographic Concepts
Projection

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

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

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

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

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

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

15. 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

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