Amanda Henley GIS Librarian Davis Library Reference September 2006 Introduction to Coordinate Systems and Working with Coordinate Systems in ArcGIS Amanda Henley GIS Librarian Davis Library Reference September 2006

…what to do? Orange County, NC

Today’s Presentation Introduction to Map Projections and Coordinate Systems Modeling the earth Geographic Coordinate Systems Datums Projected Coordinate Systems Examples of common coordinate systems Working with Coordinate Systems in ArcGIS On the fly projections “Geographic Coordinate System: GCS_Assumed_Geographic_1” Additional resources

Modeling the Earth The Earth is not a perfect sphere It is an Oblate Spheroid Different Spheroids have been devised to model the earth- they are distinguished by the length of their axes: Semi- Major Axis Minor

Geographic Coordinate Systems Locations are defined on a 3-D spherical surface Made up of graticules rather than grid cells Units are in degrees Image Source: Understanding Coordinate Systems, ESRI 2000: http://its.unc.edu/gis/arcgis/pc_documentation_83/Understanding_Map_Projections.pdf

Geographic Coordinate Systems Not uniform: Distances and measures are not accurate Meridians Converge Near Poles 1° longitude: @ Equator= 111 km @ 60° lat. = 55.8 km @ 90° lat. = 0km Distance of 60° long at equator vs. Distance of 60° long at 40° latitude Original Image Source: www.learner.org/jnorth/tm/DistanceLatLong.html

Geographic Coordinate Systems Use Decimal Degrees (angles), 3 digits or less North America: West of the Prime Meridian, so Longitude (X) is negative North of the Equator, so Latitude (Y) is positive Image Source: [http://support.esri.com/index.cfm?fa=knowledgebase.techarticles.articleShow&d=29129].

Converting between degrees, minutes seconds and decimal degrees GIS Software takes Geographic Coordinates in Decimal Degrees, not degrees, minutes, seconds Converting is easy Divide each value by the number of minutes or seconds in a degree. Example 37 degrees 36 ' 30" Divide 36 minutes by 60: 36/60=.60 Divide 30 seconds by 3600 30/3600=.00833 Add up the degrees to get the answer 37 degrees + .60 + .0083 = 37.60833 DD

Components of a Geographic Coordinate System Geographic Coordinate Systems include A Datum An angular Unit of Measure (degrees) A Prime Meridian

Datums Described Links a spheroid to a location on the earth Define the origin and orientation of the coordinate systems used to map the earth A datum is “a fixed, three-dimensional surface, an oblate spheriod, that is approximately the size and shape of the Earth. From this surface, Latitude, Longitude, and Elevation are computed”(Source Below). Source: http://www.connect.net/jbanta/FAQ.html

Datums Described, Cont. A spheroid is an earth model, a datum is a practical application of the model (souce below). A spheroid model of the earth is fixed to a base point Example: The USGS decided that the Clarke 1866 spheroid was a good approximation for the shape of the earth within the US, so they linked it to Meade’s Ranch Kansas. Thus we have NAD27 It is wrong- as you move away from the fixed point errors increase We have many datums because we keep re-measuring the earth, hopefully getting closer each time Source: http://www.connect.net.jbanta/FAQ.html

Geodetic Datums There are many datums Local: NAD 27 Datum, uses Clarke 1866 spheroid NAD 83 Datum, uses GRS 1980 spheroid Used in US and Canada Only Global WGS 84 Datum, uses WGS 1984 spheroid *Very* similar to NAD 83 In addition to being in the same projection, data themes must also be in the same datum. Source: Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder [http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html] Accessed: 01/25/06

Errors up to 1 km can result from confusing one datum for another Datum Differences May Be Difficult to See In this case, the boundaries are roughly 32 meters off: datum shifts are not uniform Errors up to 1 km can result from confusing one datum for another

Datum Differences Continued Latitude and Longitude are measurements based on a datum They are not absolute locations Latitude Longitude coordinates alone are not enough Example Davis Library: NAD83: -79.04790, 35.91097 NAD27: -79.04816, 35.91083 (almost 29 meters away)

Datum Transformations ArcGIS 9.1 NAD 1927 to NAD 1983 -for areas in the 48 contiguous states Name Code Area of Use NAD_1927_To_NAD_1983_NADCON 1241 United States (contiguous 48 states - CONUS) WGS 1984 to NAD 1983 Name Code Area of Use NAD_1983_To_WGS_1984_5 1515 United States There are five transformations to chose from, each is appropriate for different areas. 5 Supercedes 4.

Data in a Geographic Coordinate System: United States

Data in a Geographic Coordinate System: North Carolina

Projected Coordinate Systems Projected Coordinate Systems mathematically transform the 3 dimensional earth so that it can be modeled in 2 dimensions. This results in distortion Different projections are used for different areas and purposes

Different Projections Preserve Different Properties of the Earth Directions Distances Shapes Areas If some properties are maintained, errors in others may be exaggerated

Commonly Used Projected Coordinate Systems State Plane- a coordinate system that divides the United States, Puerto Rico and U.S. Virgin Islands into >120 zones. North Carolina State Plane Meters, NAD83 is used by the North Carolina CGIA NC State Plane Feet, NAD83 is used by most local data providers.

U.S. State Plane Zones Image Source: http://www.warnercnr.colostate.edu/class_info/nr502/lg3/datums_coordinates/spcs.html, Accessed January 27, 2006.

Georgia State Plane Meters East and West

North Carolina State Plane Meters and Feet

Commonly Used Projected Coordinate Systems Cont. UTM- Universal Transverse Mercator divides the globe into 60 zones, each 6° longitude. Transverse Mercator is accurate for narrow zones Often used for large scale scientific mapping Units are in meters

UTM Zones in North Carolina

Spatial Data for Orange County, NC in two different UTM Zones

Commonly Used Projected Coordinate Systems Cont. Albers Equal Area Conic: “Used by USGS for maps showing the conterminous United States (48 states) or large areas of the United States. Well suited for large countries or other areas that are mainly east-west in extent and that require equal-area representation. Used for many thematic maps.” Source: USGS http://erg.usgs.gov/isb/pubs/MapProjections/projections.html Accessed, 1/27/06.

Albers Equal Area Conic

Data in Albers Equal Area Conic and Geographic Coordinate System

Working With Coordinate Systems in ArcGIS On the Fly Projection “Geographic Coordinate System: GCS_Assumed_Geographic_1” Demonstrations Defining a Layer’s Coordinate System Using ArcCatalog Defining a Layer’s Coordinate System Using ArcToolbox Projecting a layer in ArcToolbox Projecting a layer in ArcMap