Describing Where: spatial referencing systems

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Describing Where spatial referencing systems

• Many common applications use only a simple
  reference grid

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Geographic coordinates (Latitude and
Longitude). Simple conversion of angles at the
earths center gives a basic equirectangular
projection Common and simple
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• 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

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A complicating factor the Earth is not really
round. It is in fact an oblate spheroid.
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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
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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 have to define WHERE the
sphereoid is centered!
http//www.connect.net/jbanta/FAQ.htmlAn
outstanding site that discusses sphereoid and
datum
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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 offest between projections with different
  datum can be significant
• If you are aware of the issue, it is usually
  fairly easy to find the tools to make the
  conversion

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While the Earth is roundish, maps/display
screens are FLAT Map Projections are different
ways that a curved surface can be displayed FLAT.
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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.
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The direction of projection can be changed
according to the needs of the person using the
data
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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.
http//www.colorado.edu/geography/gcraft/notes/map
proj/mapproj_f.html
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The UTM Grid (Universal Transverse Mercator)
Projection Each Cell is 6 degrees of longitude
and 8 degrees of latitude. For civilian
applications, the critical issue is the zone.
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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.
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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

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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
Alabama uses UTM
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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

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Land use patterns in Ohio from Public Land Survey
System (PLSS) survey.
Land use patterns in Ohio from uncontrolled
survey.
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USPLSS baselines for the US note the patterns of
history and politics evident in the locations of
this system.
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TN-AL Border
Huntsville AL
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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 section160 acres.
1 mile .6km 1hectare 2.2 acres
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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
• Here Alabama west state plane

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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.

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Arc-GIS supposedly, automatically, co-registers
map data in different projections

• I do not trust this process.
• You MUST know the projection information for your
  data files!
• Even if you come to rely on the automatic
  co-registration in Arc-GIS sometime it will not
  work and you MUST be able to correct the process!

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Projecting in ArcviewYou need to activate an
extension
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Getting extensions into Arcview

• There are an incredible number of extensions
  available for use with arcview.
• There are a complete set of optional extensions
  that come with arcview
• Projector! Is found in the sample directory
• The .avx file must be placed in the ext32
  directory

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The samples Directory contains useful extensions
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ArcScripts on the webhttp//arcscripts.esri.com/
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1988 scripts available for download!
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The projector! Tool is my personal favorite.
IF the initial data is in geographic projection
dd lat-long You are only asked for the OUTPUT
projection If the initial data are projected, you
must specify the INPUT and OUTPUT
projection Fairly simple and robust
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The projection utility wizard packaged as part
of v.3.2
SLOW Again, requires you know both the INPUT and
OUPTUT extensions! Handles an extremely diverse
set of projections, and datum shifts etc. If you
have complex data projection requirements, this
may be your best choice.
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Projections can be a pain in the !

• KNOW the projection you have and the projection
  you want.
• Metadata is your friend!
• Be skeptical of automated processes
• Be patient, there is no quick way to do this job
• Choose your projection based on project needs
  and organizational data standards
• raster data is more difficult to project. in
  general, project vector data to match the raster
  data.

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Metadata data about data

• Standard descriptions of data types/formats/projec
  tions etc
• http//www.fgdc.gov/metadata/metadata.html
• All federal data is required to have metadata
• ALL data should have metadata! (that includes
  data you create!)

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FGDC parameters.

• Beginning date (YYYYMMDD)
• Ending date (YYYYMMDD)
• Currentness reference
• Progress
• Intended data set maintenance and update
  frequency
• West bounding coordinate (-DDD.XXX)
• East bounding coordinate (-DDD.XXX)
• North bounding coordinate (DD.XXX)
• South bounding coordinate (DD.XXX)
• Theme keywords
• Theme Reference
• Place keywords
• Place Reference
• Limits on data accessibility

• Metadata Elements for Each Entry
• General Identification Information
• Identity of this entry (for future update)
• Originator
• Publication date (YYYYMMDD)
• Title of data set
• Edition
• Presentation Form
• Publication place
• Publisher
• Online linkage (URL)
• Abstract
• Purpose
• Supplemental Information

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• Distribution organization
• Distribution contact position/person
• Address type
• Address
• City
• State or province
• Postal code
• Country
• Phone
• Fax
• E-mail
• Dataset name as known by Distributor
• Liability held by distributor
• Date of last metadata entry or update (YYYYMMDD)

• FGDC DATA PARAMATERS contd.
• Limits on use of data
• Browse graphic URL
• Browse graphic caption
• Browse graphic file type
• Spatial data type  (vector, raster, point)
• Distribution Information

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An example of metadata

• http//www2.una.edu/gggaston/Classes/GIS/metadata.
  txt