Describing Where: spatial referencing systems

1. Describing Where: spatial referencing systems • Many common applications use only a simple reference grid

2. Geographic coordinates (Latitude and Longitude). Simple conversion of angles at the earth’s center gives a basic equirectangular projection Common and simple* (as simple as base 60 math can be!)

3. the Babylonian system is based on the number 60. Latitude and Longitude are based on a a "sexagesimal" system. A circle has 360 degrees, a degree has 60 min. and each minute is divided into 60 seconds. This nomenclature is known as DMS (degrees, minutes, seconds) • Perhaps a better system is to convert the minutes/seconds to a decimal part of a degree this is known as DD (decimal degrees). e.g. 120 30’ 00” = 120.5

4. A complicating factor: the Earth is not really ‘round’. It is in fact an ‘oblate spheroid’*.

5. Actually, the earth could be better pictured as a ‘lumpy potatoe’(my tribute to Dan Quayle) spinning through space. The Mathematical sphere and ellipsoid (sphereoid) are still open to “interpretation”

6. The picture is yet a little MORE complex! Not only do we have a lumpy blob of a planet, AND we have to assume some mathematical construct to calculate locations We also have to define WHERE the sphereoid is centered! http://www.connect.net/jbanta/FAQ.html An outstanding site that discusses sphereoid and datum

7. Datum • The easiest to use are datum that use the center of the earth (WGS84,NAD83,GRS80) as a reference. • Older data in the US often use NAD27 (North American Datum 1927). The datum for this projection is the geographic center of the US (Meads Ranch KS) • The offset between projections with different datum can be significant (100’s to 1000’s of meters) • If you are aware of the issue, it is usually fairly easy to find the tools to make the conversion

8. While the Earth is ‘roundish’, maps/display screens are FLAT Map Projections are different ways that a curved surface can be displayed FLAT.

9. Distortion: It is impossible to project a curved surface to a flat display without causing distortion of the features. An almost unlimited number of projections have been developed for the purposes of individual users.

10. A projection can intersect the surface in many different places. There is no distortion at the points of intersection… distortion increases as the distance from the intersecting points increases. The conic projection below has two lines of tangency… two lines where the distortion due to projection is zero.

11. The direction of projection can be changed according to the needs of the person using the data

12. The UTM Grid (Universal Transverse Mercator) Projection (a cylindrical projection where the ‘tube’ is pushed through the earth from side to side (the transverse part) Each Cell is 6 degrees of longitude and 8 degrees of latitude. For civilian applications, the critical issue is the zone.

13. UTM zones for the Conterminous US: Note that most states are split between 2 or 3 zones… it is critical to know which zone your data are in. Nevada is one of the few states where virtually all state data are found in UTM coordinates.

14. The “State Plane” Coordinate System • Each state has a unique set of coordinates • ‘wide’ states use a Lambert projection • ‘tall’ states use UTM projection • Larger states have multiple zones • The goal is to create minimal distortion between the curved surface and flat display and to have simple, positive coordinates

15. Wisconsin has 3 zones and uses a Lambert conic projection. Oregon has 2 zones and also uses the Lambert projection. Illinois uses a UTM projection

16. Public Land Survey System • The idea of Thomas Jefferson • Set up in 1785 for the ‘western’ US • Creates ‘square’ landuse patterns. • Commonly used in parcel descriptions

17. Land use patterns in Ohio from Public Land Survey System (PLSS) survey. Land use patterns in Ohio from ‘uncontrolled’ survey.

18. The differences between areas that were surveyed using PLSS as opposed to ‘uncontrolled surveys’ shows up commonly in the US and indicates pretty clearly WHEN and how the lands were settled. The example below is from Northern Alabama… it is a mosaic of TIGER road files from adjoining counties. In the north, the state of Tennessee was settled using the uncontrolled survey… across the line in Alabama, the lands (and roads) used the ‘township and range’ format of the PLSS.

19. SPSS baselines for the US… note the patterns of history and politics evident in the locations of PLSS.

20. N-S divisions are ‘TOWNSHIPs’ E-W divisions are ‘RANGE’ Each section is 1 mile square (640 Acres) The various ‘homestead’ acts gave rights of claim to a ¼ section…160 acres. 1 mile = .6km 1hectare = 2.2 acres

21. The Bottom line: • There are an almost unlimited number of ways that spatial data can be projected. • The most common in the US are probably: • Lat-long • UTM (universal transverse mercator) • “Albers” conic projection • Township and Range

22. You must be aware that there are lots of possibilities You must know the terms. Be prepared, data almost NEVER overlays the first time. Never hesitate to look for help… this is NOT easy stuff, and even experts are often not truly ‘expert’.

23. Projecting data in Arcview • Arcview V.3.2 and later has a integrated “projector wizard” • A simpler tool can be found in the ESRI/arcview/samples

24. The projection utility wizard The projection utility wizard is an extension that must be activated in the list of available extensions….

25. Step one….choose the shape files to be projected Step 2… identify the projection of the data to be projected…*

26. The importance of projection information cannot be over emphasized!YOU MUST know the projection parameters that you are coming from AND the projection parameters you want the data to have in the final phase!

27. Also useful to note: all the data provided by ESRI the sample shape files, TIGER files etc. are all distributed in “geographic” projection in Decimal Degreed (dd). ESRI products are comfortable with this convention and these data are a native format… knowing this makes it fairly easy to project these data to any other projection!

28. The projection utility wizard contd. In step 3 you select the new projection parameters for the map data… this wizard has literally thousands of choices…. Most of which are unlikely to ever be of use!

29. Example contd. Step 4 allows you to put the new shape file in a specfic directory. I recommend names that reflect the nature of the data… in this example I am projecting Alabama county outlines to the state plane coordinates NAD 27, Alabama West

30. The final step summarizes what you are about to do…..

31. The counties of Alabama before projection… note the XY location numbers These are extremely useful in watching the progress of a projection operation (and are often useful in deciphering projections with poor meta data)

32. The projected map of Alabama counties… ‘taller and slimmer’ … and has a completely different set of XY values to describe each place They are now in “state plane feet” rather than decimal degrees

33. The projector! extension • My personal favorite tool for projecting data as it is extremely simple and fairly robust • This is an extension that comes with arcview… but is not immediately available… • To use the projector! Extension… you first need to see how extensions are used in Arcview. • See the powerpoint called “Using Extensions”