1. CORSE '07
Spatial Data
Spatial data comes in many forms. So How does a GIS work with the data so that it can put the data in the right place on a map?

2. Outline Kinds of spatial data
Vector data
Points, lines, and polygons
Highway maps
Raster data and image data
Raster representation of data
Satellite data (Landsat)
Aerial photographs
Digital elevation data
Real world problems associated converting data from earth’s spherical coordinates to coordinates on a flat surface

3. Compare Raster & Vector
Vector GIS
Objects represented by:
points
lines
polygons
large database each object
Raster GIS
AREA represented by:
Grid cells
one value per cell
Large number thematic layers
Forest road
Cropland stream
Looks like a
map
Looks like what?
324

4. “Kinds” of GI data Vector Data
2’ resolution rectified aerial photograph
“Kinds” of GI data
Vector Data
Line
Area
Point
Aerial Photo
Landsat 7 image

5. The Digital Elevation Raster

6. Uses
Vector data is most common because you can tie huge databases to features
BUT
Raster data is very good for continuous surfaces like
Elevation
Images (dumb picture) data

7. OK…
Data on the globe is not very useful because you can’t put the globe in your report
So you need a 2D map
Thus you have to convert 3D data to 2D
Lets start with the spherical earth …

8. Syracuse
W 43.07N
Where are we?

9. Earth’s Coordinate System
X is Longitude and is measured E and W from Greenwich, England. West is negative, East is positive
Y is latitude and is measured N and S from the equator. North is positive and S is negative.
When speaking of Longitude & Latitude coordinate system we usually say “Latitude and Longitude” which is the same thing as referring to X,Y coordinates as “Y,X”. This can cause students to be confused when working in the computer when it asks for X and Y coordinates – they sometimes put in Latitude and Longitude (in that order). Things do not come out in the right place if they do that.
These are called Geographic Coordinates

10. X, Y = Longitude, Latitude90
Stretch the top
Stretch the bottom 60 30
Equator
-30
-60
The coordinate system in Longitude and Latitude (X and Y) can be drawn as a Cartesian coordinate system. Note that areas W of the prime meridian are negative
-90
-180
-90
+90
+180
Lines of constant Longitude
Lines of constant Latitude

11. X, Y = Longitude, Latitude
-76.57° °
W76.57° N42.93°
90E, 30N
+90, +30 90 60 30
Equator
-30
-60
The coordinate system in Longitude and Latitude (X and Y) can be drawn as a Cartesian coordinate system. Note that areas W of the prime meridian are negative
90W, 30S
-90
-180
-90
+90
+180
Lines of constant Longitude
Lines of constant Latitude

12. The world in Geographic CoordinatesIs
Antarctica
Really that big?

13. 3D to 2D
Geographic coordinates introduce too much distortion to be useful
So we need to convert 3D coordinates into 2D coordinates
But, there is a problem…

14. The Problem

15. What happens when a spherical, 3D surface is flattened – which we must do to put the surface on a piece of paper, a 2D surface

16. Oops!
Can't flatten a sphere
without distortion

17. The Mercator Projection
Making a Projection
Mercator can be visualized by slipping a Tube down over the earth and projecting

18. Some Projections Which is Right? Wrong Question! They are ALL right,
just different

19. Some Projections
It is pretty obvious that if you have data in different projections they are NOT going to “line up” with each other

20. Why different Projections?
Spherical coordinates to flat surface produces distortions in:
Shape
Area
Distance
Direction
So different projections for different purposes (Mercator for transportation (rumb line stright)

21. That means…
Data in different projections will not line up or be congruent!
This is something you have to be aware of …
HOWEVER
ArcGIS will project on the fly so the problem is not great
But only IF there is a metadata file for the data.
Data about Data

22. Projections: Distortion
Shape: If shapes look the same on the map and on the globe then the projection is conformal
Area: If area is preserved then you have an equal area map
Distance: If distance is preserved then the map is of uniform scale and you have an equidistance map.
Direction: maps If directions from a central location to all other points are correct then the map is Azmuthal

23. Projections: Distortion
Shape: If shapes look the same on the map and on the globe then the projection is conformal
Area: If area is preserved then you have an equal area map
Distance: If distance is preserved then the map is of uniform scale and you have an equidistance map.
Direction: maps If directions from a central location to all other points are correct then the map is Azmuthal
See your Mercator poster

24. Lets make life a bit more difficult
In addition to the many projections that 2D data can be in…
There are two Coordinate Systems that are in common use …
For smaller areas (like ½ a state)
Much of the data you will find useful will be in one of these systems

25. These systems are…
The UTM coordinate system or Universal Transverse Mercator coordinate system
And
The State Plane coordinate system
Unique to each state

26. UTM Coordinate Systems
The UTM Coordinate system is –
based on the Mercator projection
A world wide system
Except that the cylinder is now horizontal and so is tangent to the earth along a meridian which passes through the Poles
Central Meridian
Errors are Zero!

27. UTM coordinate system
Is a projected coordinate system that divides the world into 60 north and south zones, each six degrees wide.
Why bother?
Increase Accuracy and decrease distortion
Because all the data for a zone is within 3 degrees of the Central meridian it is pretty accurate!
Can’t map within multiple zones
New York is usually mapped in one zone

28. Most of NY is in UTM Zone 18
UTM Zones

29. UTM Coordinates Northing(Y) Easting(X)
The units in UTM are usually Meters
The coordinates are Eastings & Northings
The zone has to be specified
Example: Location of CCC is:
373,800 Meters E & 4,756,000 Meters N
in Zone 18, N

30. The State Plane Coordinate System
A projected coordinate system used in the United States
Divides each state into one or more zones
Also known as SPCS and SPC.

31. State Plane
Horizontal zones (Tenn) are in Lambert Conformal projections
Vertical zones are in Transverse Mercator projections
Each state has its own origins for its own system
States may have multiple zones in different projections
UNITS are usually feet BUT NOT ALWAYS

32. State Plane Zones NY East Zone 4801 NY Central Zone 4826 NY West
NY Long Island
Zone 4876

33. State Plane Zones Transverse Mercator Lambert Conformal NY East
NY Central
Zone 4826
NY West
Zone 4851
Transverse Mercator
NY Long Island
Zone 4876
Lambert Conformal

34. Another niggling Problem
The earth is only approximately spherical
We can mathematically convert features on the 3D earth to a 2D map easily if the surface is spherical and smooth
Oops - earth is pear shaped and rough
So we have to introduce the idea of a datum

35. Spheroids & Datums
A spheroid can be moved mathematically to fit different parts of the earth…
Earth
FIT
Fit
Now we have 2 different datums
Spheroid

36. So what?
The spatial properties of a GIS data layer specify both the projection or Coordinate system and the Datum
Different datums will cause shifts in location of the order of 100 meters
Not big but troublesome
In ArcGIS on-the-fly projection takes care of both projection and datum

37. Some Datums NAD 27 NAD 83 WGS 84 These are the common datums
For Coordinate Systems the spatial properties are given in statements like…
NAD_27_UTM _ZONE_18N
NAD_83_SPC _ZONE_4826

38. Summary There are a variety of spatial data types
Spherical Geographic Coordinate Systems are based on Spheroids
Spherical data is projected onto 2D maps
There are many Projections
More commonly, you will run into the class of Projections called Coordinate Systems (UTM, SP)
Projected data is based on a datum and data in different datums will not (usually) line up!

39. Summary
The subject of projections and datums is the most confusing and complex area of using GIS.
Take good notes and do your best to understand it.
At GIS conferences sessions on this topic are always very crowded! That tells you something!

40. Appendix
These slides will not be projected and are provided as a resource

41. Acronyms NAD – North American datum GCS – Geographic Coordinate System
WGS – World Geodetic System
UTM – Universal Transverse Mercator corrdinate sysem
SPC – State Plane coordinate system
GRS –Geodetic Reference System
DD – Decimal Degrees
DMS – Degrees, minutes, seconds
HARN – High Accuracy Reference Network (State Level)
NADCON – North American Datum Conversion between NAD27 & 83

42. Definitions
Vector data. Data made up of points, lines, and areas or polygons.
Raster data, Data which represents the earth’s surface and/or what is on it as a collection of cells, usually square.
Longitude – The X in spherical coordinates
Latitude – The Y ins spherical coordinates
Meridian – Great Circle passing through both poles
Eastings and Northings – X and Y in UTM & SPC