Reference Systems Reference Systems (Projections, Datums, (Projections, Datums,

Coordinates) and Coordinates) and SurveysSurveys

Source: Peter H. Dana, The Geographer's Craft Project, Department of Source: Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at BoulderGeography, The University of Colorado at Boulder, ,

http://www.ncgia.ucsb.edu/education/curricula/giscchttp://www.ncgia.ucsb.edu/education/curricula/giscc used with used with permissionpermission

ProjectionsProjections

World’s not flat (despite what World’s not flat (despite what you have heard from Dr. K!)you have heard from Dr. K!)

We want to tie our plane We want to tie our plane surveys to global systemssurveys to global systems

Submeter accuracySubmeter accuracy GPSGPS Satellite imagerySatellite imagery

ProjectionsProjections

ConformalitConformality y

Distance Distance Direction Direction Scale Scale Area Area

Lat-LongLat-Long

(unprojected)(unprojected)

MercaturMercatur LambertLambert

Scale true on 2 parallelsScale true on 2 parallels

Secant at 45°Secant at 45° (minimizes shape (minimizes shape

distortion)distortion)

Note shift of latitude linesNote shift of latitude lines(minimizes area (minimizes area exaggeration)exaggeration)

Constant azimuth for Constant azimuth for lineslines

Preserve distance along great Preserve distance along great circlecircle

PseudocylindricalPseudocylindrical

North American North American ProjectionsProjections

Equal Area Equal Dist.Equal Area Equal Dist.

Iowa State Plane USGS Iowa State Plane USGS topostopos

Similar to Gall, no secantSimilar to Gall, no secant

State Systems (hybrids)State Systems (hybrids)

OriginOrigin 31:10 North31:10 North 100:00 West 100:00 West

standard standard parallelsparallels

27:25 North27:25 North 34:55 North 34:55 North

Iowa DOT Lambert Iowa DOT Lambert HybridHybrid

DatumsDatums

1.1. Define the shape of the earthDefine the shape of the earth

2.2. Range from flat-earth to complexRange from flat-earth to complex

3.3. Wrong datum may produce 100s of Wrong datum may produce 100s of meters in errormeters in error

cartography, surveying, navigation, cartography, surveying, navigation, and astronomy, geodesy and astronomy, geodesy

Geometric Earth ModelsGeometric Earth Models

Early ideas of the figure of the earth Early ideas of the figure of the earth resulted in descriptions of the earth as resulted in descriptions of the earth as an oyster (The Babylonians before 3000 an oyster (The Babylonians before 3000 B.C.), a rectangular box, a circular disk, a B.C.), a rectangular box, a circular disk, a cylindrical column, a spherical ball, and a cylindrical column, a spherical ball, and a very round pear (Columbus in the last very round pear (Columbus in the last years of his life). years of his life).

You are here!You are here!

Geometric Earth ModelsGeometric Earth Models

Flat earth models Flat earth models are still used for are still used for plane surveying, plane surveying, over distances over distances short enough so short enough so that earth that earth curvature is curvature is insignificant (less insignificant (less than 10 kms). than 10 kms).

Looks like Looks like a a

sphere, sphere, but flat but flat

here, here, and and

herehere

The best ellipsoidal models can represent the shape of the earth over the smoothed, averaged sea-surface to within about one-hundred meters.

Sea levelSea level: average surface of the oceans ( is : average surface of the oceans ( is far more complex)far more complex)

Tidal forces and gravity cause surface to vary Tidal forces and gravity cause surface to vary by hundreds of meters!by hundreds of meters!

Gravity modelsGravity models and and geoidsgeoids are used to are used to represent local variations in gravity that represent local variations in gravity that change the local definition of a level surface change the local definition of a level surface

Geodetic HeightGeodetic Height

Coordinate Systems Coordinate Systems

Based on …Based on … DatumsDatums UnitsUnits ProjectionsProjections Reference systemsReference systems

Note false Note false eastingeasting

False False northing in northing in southern southern hemispherehemisphere

UTMUTM

Local AdjustmentsLocal Adjustments

May need a scaling factor to make May need a scaling factor to make total station measurements match total station measurements match regional coordinate systemsregional coordinate systems

e.g., Iowa DOT develops a scaling e.g., Iowa DOT develops a scaling factor for each projectfactor for each project

Based on an accurately measured point Based on an accurately measured point in the center of the projectin the center of the project

Not using a scaling factor can produce Not using a scaling factor can produce a 12’ error 30 miles from project centera 12’ error 30 miles from project center

Public Land Rectangular Surveys (USPLS)

• Townships, square with six miles on each side, are numbered with reference to a baseline and principal meridian.

• actually, few townships are truly square due to convergence of the meridians.

• Ranges are the distances and directions from baseline and meridian expressed in numbers of townships.

• Every four townships, a new baseline is established so that orthogonal meridians can remain north oriented.

Metes and Bounds•Metes and Bounds identify the boundaries of land parcels by describing lengths and directions of a sequence of lines forming the property boundary.

Linear Referencing Linear Referencing SystemsSystems

MethodsMethods MilepostMilepost MilepointMilepoint Cogo (project coordinates)Cogo (project coordinates) Lat-LongLat-Long Projected coordinatesProjected coordinates AddressAddress Literal descriptionLiteral description

Linear Referencing Linear Referencing SystemsSystems

CAD/cartography CAD/cartography Linear DatumLinear Datum

Anchor pointsAnchor points Anchor sectionsAnchor sections Reference pointsReference points

DOT Hwy Surveys DOT Hwy Surveys

1.1. The use of photogrammetry, CAD, GPS to The use of photogrammetry, CAD, GPS to establish design controls and detailsestablish design controls and details

2.2. Survey methods:Survey methods:a)a) Ground survey: windshield, transit, level, Ground survey: windshield, transit, level,

rod, chain, EDM, total stationrod, chain, EDM, total station

b)b) GPS, DGPS, Kinematic GPS - smaller crew GPS, DGPS, Kinematic GPS - smaller crew neededneeded

c)c) Photogrammetry (with control points Photogrammetry (with control points established by a or b above), including digital established by a or b above), including digital photography, orthos, softcopyphotography, orthos, softcopy

d)d) LIDAR (Light Detection and Ranging)LIDAR (Light Detection and Ranging)

Survey TypesSurvey Types 1.1. Desk StudyDesk Study2.2. Reconnaissance/cornerstoneReconnaissance/cornerstone surveysurvey (used (used

to validate or even provide a base map of to validate or even provide a base map of culture and topography sufficient to select culture and topography sufficient to select prelim. aligns.) width usu. 0.4 – 0.6 of total prelim. aligns.) width usu. 0.4 – 0.6 of total length of project, 1”=100’ or 200’, 2-5’ contourslength of project, 1”=100’ or 200’, 2-5’ contours

3.3. Location or PreliminaryLocation or Preliminary (identify BOP, EOP, (identify BOP, EOP, and PIs, and key features (drainage, and PIs, and key features (drainage, environmental land, archeological/cultural, environmental land, archeological/cultural, traffic)traffic)

4.4. Final or Construction surveyFinal or Construction survey – centerline by – centerline by station, slope stakes, drainage, edge of station, slope stakes, drainage, edge of pavement, offset, permanent land corners, pavement, offset, permanent land corners, right-of-way markersright-of-way markers

Wright, Paul, Highway Engineering, 6th Ed. Wiley, 1996