• GCS and PCS

  • What is GCS
  • What is PCS
  • The difference between GCS and PCS

• Choose a map projection

• Change the coordinate system of a map

• Commonly Used PCS

You are part of a international search and rescue team looking for a group of injured hikers in the Australian outback. The point location you have from their satellite phone is 134.577°E, 24.006°S. You locate them at point B using your machine, but your teammates from Australian locate them at point A. So what happens?

Both location A and B in the above image are correct. A is 134.577°E, 24.006°S in one GCS (Australian Geodetic Datum 1984) and B is the same coordinate location in another GCS (World Geodetic System 1984).

A geographic coordinate system (GCS) is a reference framework that defines the locations of features on a model of the earth. It’s shaped like a globe—spherical. Its units are angular, usually degrees.

• THIS IS BECAUSE although our earth isn’t FLAT, it isn’t a perfect sphere as well!

• Each Ellipsoid (the model) is designed to approximate the Earth’s shape for one part of the planet.
• Datum defines point on ellipsoid linked to point on earth (the origin, from which all other points are calculated)
  
• Datum determines the ellipsoid model thus GCS

A projected coordinate system (PCS) is flat. It contains a GCS, but it converts that GCS into a flat surface, using math (the projection algorithm) and other parameters. Its units are linear, most commonly in meters.

• BECAUSE WE WANT TO DRAW FLAT MAPS!
• Map projection is a process guide you to display the earth surface on the flat surface of the map so the parts that are most important to your map get the least distorted.

What is the relationship and difference between GCS and PCS?

• A GCS defines where the data is located on the earth’s surface. In angular units.
• A PCS tells the data how to draw on a flat surface, like on a paper map or a computer screen. In linear units.
• A projected coordinate system (PCS) is a GCS that has been flattened using a map projection.
• Your data must have a GCS before it knows where it is on earth. There has to be GCS first, and then a PCS.
• Projecting your data is optional, but projecting your map is not. (ArcGIS Pro default projection)

• A PCS is a coordinate system. A projection is a mathematical algorithm that is used to create a PCS.

• A projection defines a set of parameter values which vary depending on the projection (false easting, central meridian, standard parallel, and so on).

• Azimuthal (Planar): uses a flat plane
• Conic: uses a cone
• Cylindrical: uses a cylinder

• All projections are distorted!

• But this distortion is not equal across all four basic spatial properties of geographical features: area, shape, distance, and direction.

• The degree of Accuracy:

  • Conformal: better angles
  • Equivalent: truer area measurements
  • Equidistant: better distance measurements

• Quick Notes on Map Projections in ArcGIS

• Which GCS you choose depends on where you are mapping.

• Which PCS you use depends on where you are mapping, but also

  • the properties of your map: preserve area, shape, distance, or direction?
  • the suitable extent of your map: world, hemisphere, continent/ocean?
  • the orientation of your map
  • the general purpose of your map: Thematic, Presentation, Topographic, Navigation, National agencies?

• If your data doesn’t have a coordinate system, or you suspect it has the wrong one, you want Define Projection.

• If your data already has a coordinate system but you wish to convert it into a different one, you want Project.

• Universal Transverse Mercator (UTM)
• State Plane

In the United States, the UTM and State Plane coordinate systems and their associated projections form the basis of many of the maps produced by the federal, state, and municipal government agencies that planners use.

• Under the Universal Transverse Mercator (UTM) Projection, the meridian is the line of tangency and the central line of each zone.
• There is no distortion at the meridian line
• Each UTM zone is of 6° of longitude, and there are total 60 UTM zones (North and South).

The process of creating UTM zones:

The distribution of UTM zones across the Globe:

• After the UTM projection, a Cartesian Coordinate System is placed on top of it, make it easier for us to measure distance!

• In order to avoid negative value within the Cartesian Coordinate System, a false easting value of 500,000 meters is applied for all regions, and a false northing value of 10,000,000 meters is applied for only regions in the southern hemisphere.

The State Plane Coordinate System (SPCS): a set of 125 (not including one for Puerto Rico and US Virgin Island) geographic zones designed for specific regions of the United States.

• Each state contains one or more state plane zones, the boundaries of which usually follow county and state lines.
• There are 110 zones in the contiguous US, with 10 more in Alaska, 5 in Hawaii, and one for Puerto Rico and US Virgin Islands.
• Distortion over small areas is minimal (high level of accuracy)

• North-south orientation: Transverse Mercator Projection
• East-west orientation: Lambert Conformal Conic Projection

For Tompkins County: NAD_1983_StatePlane_New_York_Central_FIPS_3102_Feet

Universal Transverse Mercator (UTM)

• No distortion along central meridian
• Customized UTM zones for every 6°
• False easting and northing to aviod negtive values

State Plane

• Each state plane zone has its own projection based on orientation
• Distortion over small areas is minimal
• Zone boundaries follow state and county boundaries

• Map Projection Transitions
• Mercator Puzzle

Map projection animations

Geographic vs Projected Coordinate Systems

Coordinate systems, projections, and transformations

Maantay, J., & Ziegler, J. (2006). GIS for the Urban Environment. Redlands, CA: Esri Press.

Intro: Getting to Know ArcGIS Pro and Basic Mapping

1. What is GIS
2. Operating ArcGIS Pro
3. Thematic Mapping
4. Map Projection

Going forward: Geo-spatial data management