fundamentals of gis lecture materials by austin troy © 2008, except where noted lecture 6:...

Fundamentals of GIS

Lecture materials by Austin Troy © 2008, except where noted

Lecture 6:Introduction to Projections and

Coordinate Systems

By Austin Troy, University of Vermont,with sections adapted from ESRI’s online course on projections

------Using GIS--

Fundamentals of GIS

©2008 Austin Troy

The Earth’s Shape and Size•It is only comparatively recently that we’ve been able to say what both are

•Estimates of shape by the ancients have ranged from a flat disk, to a cube to a cylinder to an oyster.

•Pythagoras was the first to postulate it was a sphere

•By the fifth century BCE, this was firmly established.

•But how big was it?

Fundamentals of GIS

©2008 Austin Troy

The Earth’s Size•It was Posidonius who used the stars to determine the earth's circumference. “He observed that a given star could be seen just on the horizon at Rhodes. He then measured the star's elevation at Alexandria, Egypt, and calculated the angle of difference to be 7.5 degrees or 1/48th of a circle. Multiplying 48 by what he believed to be the correct distance from Rhodes to Alexandria (805 kilometers or 500 miles), Posidonius calculated the earth's circumference to be 38,647 kilometers (24,000 miles)--an error of only three percent.” More info -source: ESRI

http://www.uvm.edu/~atroy/nr143/Eratosthenes.htm

Fundamentals of GIS

©2008 Austin Troy

So, what shape IS the earth?•Earth is not a sphere, but an ellipsoid, because the centrifugal force of the earth’s rotation “flattens it out”.

•This was finally proven by the French in 1753

•The earth rotates about its shortest axis, or minor axis, and is therefore described as an oblate ellipsoid

Source: ESRI

Fundamentals of GIS

©2008 Austin Troy

And it’s also a….•Because it’s so close to a sphere, the earth is often referred to as a spheroid: that is a type of ellipsoid that is really, really close to being a sphere

•These are two common spheroids used today: the difference between its major axis and its minor axis is less than 0.34%.

Source: ESRI

Fundamentals of GIS

©2008 Austin Troy

Spheroids•The International 1924 and the Bessel 1841 spheroids are used in Europe while in North America the GRS80, and decreasingly, the Clarke 1866 Spheroid, are used•In Russia and China the Krasovsky spheroid is used and in India the Everest spheroid

Fundamentals of GIS

©2008 Austin Troy

Spheroids•Note how two different spheroids given slightly different major and minor axis lengths

Source: ESRI

Fundamentals of GIS

©2008 Austin Troy

Spheroids•One more thing about spheroids: If your mapping scales are smaller than 1:5,000,000 (small scale maps), you can use an authalic sphere to define the earth's shape to make things more simple

•For maps at larger scale (most of the maps we work with in GIS), you generally need to employ a spheroid to ensure accuracy and avoid positional errors

Fundamentals of GIS

©2008 Austin Troy

Geoid•While the spheroid represents an idealized model of the earth’s shape, the geoid represents the “true,” highly complex shape of the earth, which, although “spheroid-like,” is actually very irregular at a fine scale of detail, and can’t be modeled with a formula (the DOD tried and gave up after building a model of 32,000 coefficients)

•It is the 3 dimensional surface of the earth along which the pull of gravity is a given constant; ie. a standard mass weighs an identical amount at all points on its surface

•The gravitational pull varies from place to place because of differences in density, which causes the geoid to bulge or dip below or above the ellipsoid

•Overall these differences are small~100 meters

Fundamentals of GIS

©2008 Austin Troy

Geoid

www.esri.com/news/arcuser/0703/geoid1of3.html

•The geoid is actually measured and interpolated, using gravitational measurements.

http://www.esri.com/news/arcuser/0703/geoid1of3.html

Fundamentals of GIS

©2008 Austin Troy

Spheroids and Geoids•We have several different estimates of spheroids because of irregularities in the earth: there are slight deviations and irregularities in different regions

•Before remote satellite observation, had to use a different spheroid for different regions to account for irregularities (see Geoid, ahead) to avoid positional errors

•That is, continental surveys were isolated from each other, so ellipsoidal parameters were fit on each continent to create a spheroid that minimized error in that region, and many stuck with those for years

Fundamentals of GIS


The Geographic Graticule/Grid•Once you have a spheroid, you also define the location of poles (axis points of revolution) and equator (midway circle between poles, spanning the widest dimension of the spheroid), you have enough information to create a coordinate grid or “graticule” for referencing the position of features on the spheroid.

Fundamentals of GIS


The Geographic Graticule/Grid•This is a location reference system for the earth’s surface, consisting of:

•Meridians: lines of longitude and

•Parallels: lines of latitude

Source: ESRI

•Prime meridian is at Greenwich, England (that is 0º longitude)

•Equator is at 0º latitude

Fundamentals of GIS


The Geographic Graticule/Grid•This is like a planar coordinate system, with an origin at the point where the equator meets the prime meridian

•The difference is that it is not a Grid because grid lines must meet at right angles; this is why it’s called a graticule instead

•Each degree of latitude represents about 110 km, although, that varies slightly because the earth is not a perfect sphere

Fundamentals of GIS


The Geographic Grid/Graticule

•Latitude and longitude can be measured either in degrees, minutes, seconds (e.g. 56° 34’ 30”); minutes and seconds are base-60, like on a clock

•Can also use decimal degrees (more common in GIS), where minutes and seconds are converted to a decimal

•Example: 45° 52’ 30” = 45.875 °

Introduction to GIS

Fundamentals of GIS


The Geographic Grid/Graticule•Latitude lines form parallel circles of different sizes, while longitude lines are half-circles that meet at the poles

•Latitude goes from 0 to 90º N or S and longitude to 180 º E or W of meridian; the 180 º line is the date line

Source: ESRI

Fundamentals of GIS


Datums•Three dimensional surface from which latitude, longitude and elevation are calculated

•Allows us to figure out where things actually are on the graticule since the graticule only gives us a framework for measuring, and not the actual locations

•Frame of reference for placing specific locations at specific points on the spheroid

•Defines the origin and orientation of latitude and longitude lines.

Fundamentals of GIS


Datums •A datum is essentially the model that is used to translate a spheroid into locations on the earth

•A spheroid only gives you a shape—a datum gives you locations of specific places on that shape.

•Hence, a different datum is generally used for each spheroid

•Two things are needed for datum: spheroid and set of surveyed and measured points

Fundamentals of GIS


Surface-Based Datums•Prior to satellites, datums were realized by connected series of ground-measured survey monuments

•A central location was chosen where the spheroid meets the earth: this point was intensively measured using pendulums, magnetometers, sextants, etc. to try to determine its precise location.

•Originally, the “datum” referred to that “ultimate reference point.”

•Eventually the whole system of linked reference and subrefence points came to be known as the datum.

Introduction to GIS

Fundamentals of GIS


Surface Based Datums•Starting points need to be very central relative to landmass being measured•In NAD27 center point was Mead’s Ranch, KS (why?)•NAD 27 resulted in lat/long coordinates for about 26,000 survey points in the US and Canada.

•Limitation: requires line of sight, so many survey points required

•Problem: errors compound with distance

Fundamentals of GIS


c

Surface Based Datums•These were largely done without having to measure distances. How?

•Using high-quality celestial observations and distance measurements for the first two observations, could then use trigonometry to determine distances.

ab

A

With b and c and A known, we can determine a’s location through solving for B and C by the law of sines

B=A(sin(b))/(sin(a))

B

C

D

E

Mead’s Ranch

Secondary Measured point

Fundamentals of GIS


Satellite Based Datums•With satellite measurements the center of the spheroid can be matched with the center of the earth.

•Satellites started collecting geodetic information in 1962 as part of National Geodetic Survey

•This gives a spheroid that when used as a datum correctly maps the earth such that all Latitude/Longitude measurements from all maps created with that datum agree.

•Rather than linking points through surface measures to initial surface point, are measurements are linked to reference point in outer space

http://www.ngs.noaa.gov/faq.shtml#WhatNAD

Fundamentals of GIS


Common Datums•Previously, the most common spheroid was Clarke 1866; the North American Datum of 1927 (NAD27) is based on that spheroid, and has its center in Kansas.

•NAD83 is the new North American datum (for Canada/Mexico too) based on the GRS80 geocentric spheroid. It is the official datum of the USA, Canada and Central America

•World Geodetic System 1984 (WGS84) is newer spheroid/datum, created by the US DOD; it is more or less identical to Geodetic Reference System 1980 (GRS80). The GPS system uses WGS84.

Introduction to GIS

Fundamentals of GIS


Lat/Long and Datums•These pre-satellite datums are surface based.

•A given datum has the spheroid meet the earth in a specified location somewhere.

•Datum is most accurate near the touching point, less accurate as move away (remember, this is different from a projection surface because the ellipsoid is 3D)

•Different surface datums can result in different lat/long values for the same location on the earth.

•So, just giving lat and long is not enough!!!

Fundamentals of GIS


Lat/Long and Datums•Lat/long coordinates calculated with one datum are valid only with reference to that datum.

•This means those coordinates calculated with NAD 27 are in reference to a NAD 27 earth surface, not a NAD 83 earth surface.

•Example: the DMS control point in Redlands, CA is -117º 12’ 57.75961”, 34 º 01’ 43.77884” in NAD 83 and -117 º 12’ 54.61539” 34 º 01’ 43.72995” in NAD 27

•Click here for a chart of the different coordinates for the Capital Dome center under different datums (Peter Dana)

http://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.html

Fundamentals of GIS


Datum Shift•NAD83 is superior to NAD 27 because:

•NAD83 is more accurate and NAD27 can result in a significant horizontal shift

•When we go from a surface-oriented datum to a spheroid-based datum, the estimated position of survey benchmarks improves; this is called datum shift

•That shift varies with location: 10 to 100 m in the cont. US, 400 m in Hawaii, 35 m in Vermont

•Click here for an example from Peter Dana

Fundamentals of GIS


Datum Shift Example

source;: http://gallery.geocaching.com.au/Maps/DatumShift andhttp://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html

Fundamentals of GIS


Map Projection•This is the method by which we transform the earth’s spheroid (real world) to a flat surface (abstraction), either on paper or digitally

•Because we can’t take our globe everywhere with us!

•Remember: most GIS layers are 2-D3D

2D

Think about projecting a see-through globe onto a wall

Source: ESRI

Fundamentals of GIS


Map Projection•The earliest and simplest map projection is the plane chart, or plate carrée, invented around the first century; it treated the graticule as a grid of equal squares, forcing meridians and parallels to meet at right angles

•If applied to the world as mapped now, it would look like:

Fundamentals of GIS


Map Projection-distortion•By definition, project distorts; four types:

•Shape

•Area

•Distance

•Direction

•Some projections specialize in preserving one or several of these features, but none preserve all

Fundamentals of GIS


Map Projection-distortion•Shape: projection can distort the shape of a feature. Conformal maps preserve the shape of smaller, local geographic features, while general shapes of larger features are distorted. That is, they preserve local angles; angle on map will be same as angle on globe. Conformal maps also preserve constant scale locally

Fundamentals of GIS


Map Projection-distortion•Area:projection can distort the property of equal area (or equivalent), meaning that features have the correct area relative to one another. Map projections that maintain this property are often called equal area map projections.

•For instance, if S America is 8x larger than Greenland on the globe will be 8x larger on map

•No map projection can have conformality and equal area; sacrifice shape to preserve area and vice versa.

Fundamentals of GIS


Map Projection-distortion•Distance: Projection can distort measures of true distance. Accurate distance is maintained for only certain parallels or meridians unless the map is very localized. Maps are said to be equidistant if distance from the map projection's center to all points is accurate. We’ll go into this more later.

Fundamentals of GIS


Map Projection-distortion•Direction:Projection can distort true directions between geographic locations; that is, it can mess up the angle, or azimuth between two features; projections of this kind maintain true directions with respect to the map projection's center. Some azimuthal map projections maintain the correct azimuth between any two points. In a map of this kind, the angle of a line drawn between any two locations on the projection gives the correct direction with respect to true north.

Fundamentals of GIS


Map Projection-distortion•Hence, when choosing a projection, one must take into account what it is that matters in your analysis and what properties you need to preserve

•Conformal and equal area properties are mutually exclusive but some map projections can have more than one preserved property. For instance a map can be conformal and azimuthal

•Conformal and equal area properties are global (apply to whole map) while equidistant and azimuthal properties are local and may be true only from or to the center of map

Fundamentals of GIS


Area Distortion

• 827,000 square miles• 6.8 million square

miles

Mercator Projection

Fundamentals of GIS


Distance distortion

•4,300 km: Robinson•5,400 km: Mercator

Fundamentals of GIS


Shape distortion

•Mercator (left)•World Cylindrical Equal Area (above)•The distortion in shape above is necessary to get Greenland to have the correct area; •The Mercator map looks good but Greenland is many times too big

Fundamentals of GIS


Distortion in specific projectionsMercator—goes on forever

robinson

sinusoidal

Fundamentals of GIS


Distortion in specific projections•Some examples:

•Mercator maintains shape and direction, but sacrifices area accuracy

•The Sinusoidal and Equal-Area Cylindrical projections both maintain area, but look quite different from each other. The latter distorts shape

•The Robinson projection does not enforce any specific properties but is widely used because it makes the earth’s surface and its features look somewhat accurate

Fundamentals of GIS


Quantifying distortion•Tissot’s indicatrix, made up of ellipses, is a method for measuring distortion of a map; here is Robinson

Fundamentals of GIS


Quantifying distortion•Here is Sinusoidal Area of these ellipses

should be same as those at equator, but shape is different

Fundamentals of GIS


Map Projection-General Types•Cylindrical projection: created by wrapping a cylinder around a globe and, in theory, projecting light out of that globe; the meridians in cylindrical projections are equally spaced, while the spacing between parallel lines of latitude increases toward the poles; meridians never converge so poles can’t be shown

Source: ESRI

Fundamentals of GIS


Cylindrical Map Types1. Tangent to great circle: in the simplest case, the

cylinder is North-South, so it is tangent (touching) at the equator; this is called the standard parallel and represents where the projection is most accurate

2. If the cylinder is smaller than the circumference of the earth, then it intersects as a secant in two places

Source: http://nationalatlas.gov/articles/mapping/a_projections.html

Fundamentals of GIS


Cylindrical Map TypesSecant projections are more accurate because projection is

more accurate the closer the projection surface is to the globe and a when the projection surface touches twice, that means it is on average closer to the globe

The distance from map surface to projection surface is described by a scale factor, which is 1 where they touch

Earth surface

Projection surface

.9996

Central meridianStandard meridians

Fundamentals of GIS


Cylindrical Map Types3. Transverse cyclindrical projections: in this type

the cylinder is turned on its side so it touches a line of longitude; these can also be tangent

Fundamentals of GIS


Cylindrical map distortion• A north-south cylindrical Projections cause major

distortions in higher latitudes because those points on the cylinder are further away from from the corresponding point on the globe

• Scale is constant in north-south direction and in east west direction along the equator for an equatorial projection but non constant in east-west direction as move up in latitude

• Requires alternating Scale Bar based on latitude

Fundamentals of GIS


Cylindrical map distortion•If such a map has a scale bar (see map in 104 Aiken), know that it is only good for those places and directions in which scale is constant—the equator and the meridians•Hence, the measured distance between Nairobi and the mouth of the Amazon might be correct, but the measured distance between Toronto and Vancouver would be off; the measured distance between Alaska and Iceland would be even further off

Fundamentals of GIS


Cylindrical Map Distortion

X miles0 ◦ atitude

25 ◦ latitude

50 ◦ latitude

Fundamentals of GIS


Cylindrical map distortion•Why is this? Because meridians are all the same length, but parallels are not.•This sort of projection forces parallels to be same length so it distorts them •As move to higher latitudes, east-west scale increases (2 x equatorial scale at 60° N or S latitude) until reaches infinity at the poles; N-S scale is constant

Fundamentals of GIS


General Projection Types: Conic•Projects a globe onto a cone

•In simplest case, globe touches cone along a single latitude line, or tangent, called standard parallel

•Other latitude lines are projected onto cone

•To flatten the cone, it must be cut along aline of longitude (see image)

•The opposite line of longitudeis called the central meridian

Source: ESRI

Fundamentals of GIS


Conic Projection•Is most accurate where globe and cone meet—at the standard parallel

•Distortion generally increases north or south of it, so poles are often not included

•Conic projections are typically used for mid-latitude zones with east-to-west orientation. They are normally applied only to portions of a hemisphere (e.g. North America)

Fundamentals of GIS


Conic Projection•Can be tangent or secant

•Secant are more accurate for reasons given earlier

Fundamentals of GIS


General Projection Types:Planar or Azimuthal

•Project a globe onto a flat plane

•The simplest form is only tangent at one point

•Any point of contact may be used but the poles are most commonly used

•When another location is used, it is generally to make a small map of a specific area

•When the poles are used, longitude lines look like hub and spokes Source: ESRI

Fundamentals of GIS


Map Projection-General Types•Planar or Azimuthal Projections:

•Because the area of distortion is circular around the point of contact, they are best for mapping roughly circular regions, and hence the poles

Fundamentals of GIS


Map Projection-Specific Types•Mercator: This is specific type of cylindrical projection

•Invented by Gerardus Mercator during the 16th Century

•It was invented for navigation because it preserves azimuthal accuracy—that is, if you draw a straight line between two points on a map created with Mercator projection, the angle of that line represents the actual bearing you need to sail to travel between the two points

Source: ESRI

Fundamentals of GIS


Map Projection-Specific Types•Mercator: Of course the Mercator projection is not so good for preserving area.

Take a look at how it enlarges high latitude features like Greenland Antarctica and shrinks mid latitude features

Fundamentals of GIS


Map Projection-Specific Types•Transverse Mercator: Invented by Johann Lambert in 1772, this projection is cylindrical, but the axis of the cylinder is rotated 90°, so the tangent line is longitudinal, rather than the equator

•In this case, only the central longitudinal meridian and the equator are straight lines All other lines are

represented by complex curves: that is they can’t be represented by single section of a circle

Source: ESRI

Fundamentals of GIS


Map Projection-Specific Types•Transverse Mercator:

•Transverse Mercator projection is not used on a global scale but is applied to regions that have a general north-south orientation, while Mercator tends to be used more for geographic features with east-west axis.

•It is used in commonly in the US with the State Plane Coordinate system, with north-south features

Fundamentals of GIS


Map Projection-Specific Types•Lambert Conformal Conic:invented in 1772, this is a form of a conic projection

•Latitude lines are unequally spaced arcs that are portions of concentric circles. Longitude lines are actually radii of the same circles that define the latitude lines.

Source: ESRI

Fundamentals of GIS


Map Projection-Specific Types•The Lambert Conformal Conic projection is very good for middle latitudes with east-west orientation.

•It portrays the pole as a point

•It portrays shape more accurately than area and is commonly used for North America.

•The State Plane coordinate system uses it for east-west oriented features

Fundamentals of GIS


Map Projection-Specific Types•The Lambert Conformal Conic projection is a slightly more complex form of conic projection because it intersects the globe along two lines, called secants, rather than along one, which would be called a tangent

•There is no distortion along those two lines

•Distortion increases as move awayfrom secants

Source: ESRI

Fundamentals of GIS


Map Projection-Specific Types•Albers Equal Area Conic projection: Again, this is a conic projection, using secants as standard parallels but while Lambert preserves shape Albers preserves area

•It also differs in that poles are not represented as points, but as arcs, meaning that meridians don’t converge

•Latitude lines are unequally spaced concentric circles, whose spacing decreases toward the poles.

•Developed by Heinrich Christian Albers in the early nineteenth century for European maps

Fundamentals of GIS


Map Projection-Specific Types•Albers Equal Area Conic: It preserves area by making the scale factor of a meridian at any given point the reciprocal of that along the parallel.

•Scale factor is the ratio of local scale a point on the projection to the reference scale of the globe; 1 means the two are touching and greater than 1 means the projection surface is at a distance

Fundamentals of GIS


Other Selected Projections• More Cylindrical equal area: (have straight meridians and parallels, the meridians are equally spaced, the parallels unequally spaced)

• Behrmann cyclindrical equal-area: single standard parallel at 30 ° north

•Gall’s stereographic: secant intersecting at 45° north and 45 ° south

•Peter’s: de-emphasizes area exaggerations in high latitudes; standard parallels at 45 or 47 °

Thanks to Peter Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder for links

Fundamentals of GIS


Other Selected Projections• Azimuthal projections:

•Azimuthal equidistant: preserves distance property; used to show air route distances

•Lambert Azimuthal equal area: Often used for polar regions; central meridian is straight, others are curved

•Oblique Aspect Orthographic

•North Polar StereographicThanks to Peter Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder for links

Fundamentals of GIS


Other Selected Projections• More conic projections

•Equidistant Conic: used for showing areas near to, but on one side of the equator, preserves only distance property

•Polyconic: used for most of the early USGS quads; based on on an infinite number of cones tangent to an infinite number of parallels; central meridian straight but other lines are complex curves


Fundamentals of GIS


Other Selected Projections• Pseudo-cylindrical projections: resemble cylindrical projections, with straight, parallel parallels and equally spaced meridians, but all meridians but the reference meridian are curves

•Mollweide: used for world maps; is equal-area; 90th meridians are semi-circles

• Robinson:based on tables of coordinates, not mathematical formulas; distorts shape, area, scale, and distance in an attempt to make a balanced map


Fundamentals of GIS


Coordinate Systems•Map projections, like we discussed in last lecture provide the means for viewing small-scale maps, such as maps of the world or a continent or country (1:1,000,000 or smaller)

•Plane coordinate systems are typically used for much larger-scale mapping (1:100,000 or bigger)

Fundamentals of GIS


Coordinate Systems•Projections are designed to minimize distortions of the four properties we talked about, because as scale decreases, error increases

•Coordinate systems are more about accurate positioning (relative and absolute positioning)

•To maintain their accuracy, coordinate systems are generally divided into zones where each zone is based on a separate map projection

Fundamentals of GIS


Reason for PCSs•Remember from before that projections are most accurate where the projection surface is close to the earth surface. The further away it gets, the more distorted it gets

•Hence a global or even continental projection is bad for accuracy because it’s only touching along one (tangent) or two (secant) lines and gets increasingly distorted

Fundamentals of GIS


Reason for PCSs•Plane coordinate systems get around this by breaking the earth up into zones where each zone has its own projection center and projection.

•The more zones there are and the smaller each zone, the more accurate the resulting projections

•This serves to minimize the scale factor, or distance between projection surface and earth surface to an acceptable level

Fundamentals of GIS


Coordinate Systems•The four most commonly used coordinate systems in the US:

•Universal Transverse Mercator (UTM) grid system

•State Plane Coordinate System (SPC)•Others:

•The Universal Polar Stereographic (UPS) grid system

•The Public Land Survey System (PLSS)

Fundamentals of GIS


UTM•Universal Transverse Mercator is a very common projection

•UTM is based on the Transverse Mercator projection (remember, that’s using a cylinder turned on its side)

•It generally uses either the NAD 27 or NAD83 datums, so you will often see a layer as projected in “UTM83” or “UTM27”

Fundamentals of GIS


UTM•UTM divides the earth between 84°N and 80°S into 60 zones, each of which covers 6 degrees of longitude

•Zone 1 begins at 180 ° W longitude.

World UTM zones

Fundamentals of GIS


UTM•US UTM zones

Fundamentals of GIS


UTM•Each UTM zone is projected separately

•There is a false origin (zero point) in each zone

•In the transverse Mercator projection, the “cylinder” touches at two secants, so there is a slight bulge in the middle, at the central meridian. This bulge is very very slight, so the scale factor is only .9996

•The standard meridians, where the cylinder touches

Fundamentals of GIS


UTM•Because each zone is big, UTM can result in significant errors as get further away from the center of a zone, corresponding to the central line

Fundamentals of GIS


UTM•Scale factors are .9996 in the middle and 1 at the secants

Earth surface

Projection surface

.9996

Central meridian

Standard meridians

Fundamentals of GIS


UTM•In the N hemisphere, UTM coordinates are measured from a false origin at the equator and 500,000 meters west of the central meridian

•In the S hemisphere, they are measured from a false origin 10,000,000 meters south of the equator and 500,000 meters west of the central meridian

•Accuracy: 1 in 2,500

Fundamentals of GIS


UTM•UTM is used for large scale mapping applications the world over, when the unit of analysis is fairly small, like a state

•For portraying large land units, like Alaska or the 48 states, a projection is usually used, like Albers Equal Area Conic

Fundamentals of GIS


UPS Grid•The Universal Polar Stereographic Grid system cover the polar areas and uses the stereographic projection, centered on the pole

•Used to divide the polar area into a series of 100,000 meter squares

Fundamentals of GIS


SPC System•State Plane Coordinate System is one of the most common coordinate systems in use in the US

•It was developed in the 1930s to record original land survey monument locations in the US

•More accurate than UTM, with required accuracy of 1 part in 10,000

•Hence, zones are much smaller—many states have two or more zones

Fundamentals of GIS


SPC System•Transverse Mercator projection is used for zones that have a north south access.

•Lambert conformal conic is used for zones that are elongated in the east-west direction. Why?

•Units of measurement are feet, which are measured from a false origin.

•SPC maps are found based on both NAD 27 and NAD 83, like with UTM, but SPC 83 is in meters, while SPC 27 is in feet

Fundamentals of GIS


SPC System•Note how a conic projection is used here, since the errors indicate an east-west central line

Polygon errors-state plane

Fundamentals of GIS


SPC System•Many States have their own version of SPC

•Vermont has the Vermont State Plane Coordinate System, which is in meters and based on NAD 83

•In 1997, VSGI converted all their data from SPC 27 to SPC 83

•Vermont uses Transverse Mercator because of its north-south orientation

Fundamentals of GIS


•Here are some State Plane zone maps

SPC System

Fundamentals of GIS


•Here are some State Plane zone mapsSPC System

fundamentals of gis lecture materials by austin troy © 2008, except where noted lecture 6:...

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