This is what earth looks like when you take a picture of it from 29,000 kilometers away.

• This photo is called the Blue Marble, it was taken from Apollo 17

This is the version that was published.

• NASA rotated the image 180 degrees to fit peoples expectations

A lumpy ball of rock flying through space.

• Despite what you may have been told
• The earth isn't round!
• It has multiple distortions caused by:
• Topography
• Gravity Differentials
• Centrifugal Force

Cartographers have developed strategies for dealing with these distortions:

• Topography
• Gravity Differentials
• Centrifugal Force

Topography is very localized, in most applications it isn't explicitly needed to make a map.

• We can account after the fact if needed
• Digital Elevation Models

Topography is very localized, in most applications it isn't explicitly needed to make a map.

• We can account after the fact if needed
• Digital Elevation Models
• Contour Lines

A simplified model of the Earth that ignores elevation differences due to topography.

• Earths crust is not uniformly dense, causes gravity differentials to form.
• Differences in density cause the surface to "sink" down or "float" up
• Continental scale undulations, +85 m (Iceland) to −106 m (southern India)

A simplified model of the Earth that ignores elevation differences due to topography.

• Gravity differences measured by satellites

A fairly close approximation of Earth's shape.

• Due to Centrifugal Force the earth is ~ 26 km wider at the equator

Measuring distance/height requires a reference point.

• A Datum is the reference system we to do the fitting
• Gives meaning to numbers/coordinates

Fitting a spheroid to the geoid allows us to minimize elevation differences due to density differentials and account for effects of Centrifugal force, while ignoring topography.

• A Datum is the reference system we to do the fitting
• Can be Global or Local

The center of the earth is used as the reference point.

• Fits the geoid fairly well everywhere

A point on the geoids surface is used as the reference point.

• Fits geoid very well in one region

Generally speaking:

• For global maps, you always need a global datum
• A local datum is better for small areas like a city
• Global datums can be used, they just might shift things a bit

Only minor differences between local and global datums in North America.

• For most applications ...
• The choice of global vs. local is not really that important
• For others, its very important
• Aviation
• Self driving cars

Latitude/Longitude is the primary Geographic Coordinate System (GSC) that we use to describe locations on the "the earth's surface"

• Location on a 3D object with just 2 numbers
• Fixed to the surface of spheroid
• Not the actual earth's surface

Sometimes refereed to as a graticule.

Degrees Minutes Seconds

• Vancouver BC: 49°15′40″N 123°06′50″W
• Sydney NSW: 33°51′54″S 151°12′34″E

Decimal Degrees

• Vancouver BC: 49.261111, -123.113889
• Sydney NSW: -33.865, 151.209444

Meridians converge at the poles!

• Distance between degrees of longitude decreases with increasing latitude
• This is the central issue with projecting a map!
• You can't accurately display a Geographic Coordinate System on a 2D surface (map/screen).

Displaying Lat/Lon in 2D doesn't work well

• Causes things to look "scrunched"

A map projection is a mathematical transformation used to flatten a GCS.

• Imagine sending rays of light through the ellipsoid onto a flat surface, the resulting image is a projection

Applying a projection:

• Converts to linear units
• Allows distance/area calculations
• Makes things look better