Exercise 2
Write one to two sentences explaining each of the following:
Ellipsoid Models
1. Clarke 1866
Clarke 1866 is the basis for the NAD 27 Datum. It is the oldest and thus the most “inaccurate”
model. It is approximately 236 meters from the center of the GRS 80 model.
2. GRS 80
The Geodetic Reference System (GRS 80) is the basis for the NAD 83 Datum. The information
was collected through satellite information, so it is much more accurate than Clarke 1866. There
is an approximately 2 meters from the WGS 84 model.
3. WGS 84
The World Geodetic System (WGS 84) is the basis for the WGS 84 Datum. The difference
between WGS 84 and GRS 80 is a slight difference in scale and rotation of angles.
Datum
4. NAD 27
The North American Datum of 1927 (NAD 27) has a common reference point at the calculated
center of the contiguous United States. Because of the reference point, the datum is most
accurate in North America. Other parts of the world are poorly represented and inaccurate.
5. NAD 83
The North American Datum of 1983 (NAD 83) does not have a single reference point because
the model it is based off satellite data collected across the world. It gives the world more
accurate representation and does not disproportionately warp one hemisphere over another.
6. WGS84
The World Geodetic System Datum (WGS 84) is very similar to NAD 83. The main difference
between the two is that WGS 84 is a slightly less flattened version of NAD 83.
Coordinate Systems
7. Geographic/Lat./Long.
The latitude/longitude coordinate system is the most commonly used coordinate system by the
public. It measures the angle from the calculated center of the earth to the point on the earth’s
surface. The system ranges from +180° to -180° longitude and +90° to -90° latitude.
8. UTM
The Universal Transverse Mercator (UTM) is a coordinate system that divides the earth into 60
zones (6° in longitude each). This coordinate system does not measure altitude, so it operates
under the assumption that Earth is a perfect ellipsoid.
9. State Plane
The State Plane is a coordinate system that only covers the United States. Each state is divided
into zones, ranging from 1-6 zones depending on the size of the state. The coordinate system
has 120 zones in total and because of the small zones, this coordinate system is quite accurate.
10. Military Grid
The Military Grid Reference System is a coordinate system that is used by NATO militaries. The
Earth is divided by a UTM grid that has a wide range of precision levels. The grid-zones are 6°
longitude, and are normally 8° high but the northernmost latitude is 12° high. Each UTM grid is
then divided into 100,000 meter squares. Each column is given a letter (A-Z, omitting I and O),
and each row is given a letter (A-V, omitting I and O). These two letters together identifying a
small area on Earth.
Projections
11. Projections – what are they? Equivalent vs. conformal, a few examples of projections
Map projection is the process of taking a 3-dimentional object and transforming it so that it can
be viewed in a 2-dimensional plane. Every map projection must distort some feature of the
object, so that it will work on a two-dimensional plane. It will distort shape, relative area,
distance, or direction. Equivalent maps will maintain the size of the features, and conformal
maps maintain the shape of the features. Compromise maps are a mix of the two and will
maintain most of the shape and most of the size but will distort each slightly. There are three
main families of map projections: Cylindrical Projections, Azimuthal/Planar Projections, and
Conic Projections. Some examples of Cylindrical Projections are: Mercator projection, Miller
projection, Plate Carrée projection, Universal transverse Mercator projection. Some examples of
Azimuthal/Planar Projections are: Lambert equal-area azimuthal projection, Orthographic
projection, Stereographic projection, Universal polar stereographic projection. Some examples
of Conic Projections are: Albers Equal-area projection, Equidistant projection, and Lambert
conformal projection.