Geography Reference
In-Depth Information
fundamental way to store data, this makes it possible to easily make maps of any desired scale on the
computer monitor or on paper. Scale is only a consideration when you measure on a physical map.
Computers take the worry out of determining scale accuracy. Because of the vast computational power
of a computer, there is no difficulty in scale conversion. The days of: “Let's see, this distance is 5.3 inches
on the map, and 1 inch is 12 miles, so the distance is about 64 miles” are over. Scale is a minor concept in
GIS—one used only on final output.
Geographic vs. Projected Coordinates: A Comparison
Advantages of the spherical coordinate system —You can represent any point on the Earth's surface as
accurately as your measurement techniques allow. The system itself does not introduce errors.
Disadvantages of a spherical coordinate system —You will encounter complex and time-consuming
arithmetic calculations in determining the distance between two points or the area surrounded by
a polygon determined by a set of points. Latitude-longitude numbers plotted directly on paper in a
Cartesian coordinate system result in distorted—sometimes greatly distorted—figures.
Advantages of a projected coordinate system on the Cartesian plane —Calculations of distances between
points are trivial. Calculations of areas are relatively easy. Graphic representations are realistic,
provided the area covered is not too large.
Disadvantages of a projected coordinate system on the Cartesian plane —Almost every point is in the wrong
place, although maybe not by much. All projections introduce errors. Depending on the projection,
these errors are in distances, sizes, shapes, or directions.
Whether you use geographic or projected coordinates, ensuring that the parameters of geographic data
match is of paramount importance in combining GIS data sets if you want the right answers! 13
Two Projected Coordinate Systems:
UTM and State Plane
A coordinate system called Universal Transverse Mercator 14 was developed based on a series of 60
projections onto semi-cylinders that contact the Earth along meridians. (To consider, for example, one
of these projections, imagine a sheet of paper curved so that it becomes a half cylinder whose radius is
that of the Earth's. Then, with the axis of the cylinder oriented in an east-west direction—hence the term
transverse—the paper is brought into contact with a globe along the meridian designating 3 o longitude.
Then, the surface of the Earth between 0 o and 6 o is projected onto the paper). This process is repeated
for central meridians of 9 o , 15 o , 21 o , and so forth up to 357 o . The term “zone” is ambiguously used for
this swath of territory. However, UTM projections are further subdivided into areas, also called zones,
covering 6 o of longitude and, for most zones, 8 o of latitude. Further, ArcGIS divides a total zone into a
13 The parameters of data sets may not always match but may be close enough so that any error introduced is trivial. In
many instances and places, for example, NAD83 and WGS84 match within centimeters. However, you must research
carefully to know when you can use data sets that don't match exactly. The Esri manual Understanding Map Projections
can help.
14 The idea of the Mercator projection was developed in 1568 by Gerhardus Mercator, a Flemish geographer,
mathematician, and cartographer.
 
 
Search WWH ::




Custom Search