1. Coordinate Systems, Projections & Introduction to Choropleth Maps
May 17, 2007
Refer to:
Krygier & Wood, pages
Monmonier, pages 5-18
Coordinate Systems, Projections & Introduction to Choropleth Maps

2. First: Rapid Review - Concepts
Labs 1 & 3: Illustrator
Pen Tool - Generalize
Closed Path Shapes
Fill, Stroke (simple)
Gradient Fill
Layers
Advanced Methods *

3. ArcMap Layout Element placement Map Layout Neatline Title/Subtitle
North Arrow
Scale *
Legend

4. Comments from last week
Type use: In cartography, using decorative type is not desirable.
Distracts from features of importance
Difficult to read
Legend
May or may not have a title, frame, may include useful information for understanding the map
Feature symbology
Adopt consistent conventions

5. LEGEND Label

7. Measuring the Earth ??
Around 2300 BP
Eratosthenes' Map

8. Erotosthenes' Calculation
Estimate – 40,500 km (25,170 mi)
Actual – 40,075 km (24,960 mi)
Not bad....there is of course dispute:
1. Disagreement on units (500 mi is itself an estimate for stadia, the unit he used)
2. Syene (Cyene) is not exactly on the same meridian as Alexandira
3. Although measured on the solstice, the sun's rays were not exactly parallel because Syene is not on the northern Tropical latitude.
Historically significant best guess...
Angle calc
on summer
solstice, noon

9. Eratosthenes' Calculation
The first acknowledged study in geodesy:
the determination of the figure and dimensions of the earth
Eratosthenes also did not realize that the Earth is NOT a perfect sphere. Until the 1600's this assumption was not in question (Newton, centrifugal forces).
A dispute ensued (Newton vs Decartes), and after two major expeditions (Lapland, Ecuador) and nine years, it was proven to be an oblate spheroid, bulging at the equatorial region.
Read: Whitaker, R., (2004). The Mapmaker's Wife

10. Geodesy & The Reference Ellipsoid
The oblate spheroid has a semimajor (a) and semiminor axis (b)
The idea of an ellipsoid describes the shape and size of the Earth
Over 30 ellipsoids exist, depending where on Earth

11. But what ARE they? They are local measurements
The ellipsoids you will encounter in the GIS world, most likely are:
WGS 84, GRS 1980 and Clark 1866

12. Geoid Somewhat abstract
The « Geoid » is a concept that has emerged after the advent of satellite technologies (post 1950's)
The Earth's surface varies – determine using spherical harmonics
Geoid

13. Geodetic Datums (or Data)
A reference point from which measurements are made. Determined by a coincident point between geoid and reference ellipsoid
Example: Clarke 1866 with single point in Kansas as reference comprises the NAD 27 (North American Datum of 1927 )

14. The Bottom Line
These are nice things to know, but what is actually relevant or important to GIS?
Satellite technology found that NAD 27 is quite erroneous, so a new reference ellipsoid, GRS 80 (Geodetic Reference System of 1980), was created
Using that ellipsoid, NAD 83 was implemented.

15. And?
These differences are often visually negligible in terms of thematic mapping
Geodesy is more important to navigation, distance and directional applications
However, these terms will come up repeatedly as you enter GIS positions and it is important to know from where they came and what they mean (generally)

16. BREAK Be back in 5 minutes
Tired?
BREAK
Be back in 5 minutes

17. Definition: Map Projection
a: systematic presentation of
intersecting coordinate lines on a
flat surface upon which features
from the curved surface of the earth
or the celestial sphere may be
mapped b: the process or
technique of reproducing a spatial
object upon a plane or curved
surface or a line by projecting its
points; also a graph or figure so
formed….
In other words…transformation of a curved surface to a flat one.
Image borrowed from the Harvard GSD Website

18. Projection Classes There are three classes of projection:
Planar (flattened)
Conic
Cylindrical

19. There are two cases
The 'tangent' case
The 'secant' case

20. Aspect

21. Consequences of Projection
DISTORTION

22. DISTORTION
You can never preserve all of these at once, but you can try.
Distance - Equidistant
Areas – Equal Area
Angles - Conformal
Shapes - Equivalent
Direction - Azimuthal

23. More DISTORTION

24. How do I choose a projection!?
A set of guidelines was developed by Snyder (1987) to help choosing a projection, depending on what you want to preserve in your map
U.S. Geological Survey Professional Paper 1453
John P. Snyder

25. Planar Geometry
The x and y axes are the drawing space for a planar map
Typically, you read units in (x,y)
The analogous Earth system is longitude and latitude

26. Earth's Geometric Grid Latitude, Longitude Northing, Easting
Northing is distance north or south from the Equator in meters
Easting is the distance from an arbitrary reference line (500,000 m west of the Prime Meridian)

27. UTM Zones

28. Military Grid System

29. GEOREF (World Geographic System)

30. State Plane System – used often
State plane systems were developed in order to provide local reference systems that were tied to a national datum.
They can be a challenge if you are crossing regions while working

31. Like in this case

32. Another break
Be back in 5 minutes

33. Scale Just a quick review Representative Fraction Not tied to units
Visual/Graphical
Statement of Equivalency

34. Finally... Introduction to Lab #4 Will span 2 weeks (due May 31st)
Choropleth Maps (lecture, next week)
ArcMap data management

35. The Choropleth Map

36. Choropleth Maps Very common Sometimes not well
May be used to illustrate change
Highly data dependent
Color scheme is important

37. More examples???
Next week, lecture will focus on Choropleth Maps, Color Choices, and more cartographic examples