Introduction to Coordinate Systems

Which of the following statements about geographic coordinate systems are true? Choose three. A geographic coordinate system gives linear measurements on a planar surface from a predefined starting point. A geographic coordinate system's coordinates are based on latitude and longitude values. A geographic coordinate system's coordinates are measured in linear units, such as feet or meters. A geographic coordinate system uses a three-dimensional spherical model to identify specific locations on the earth. A geographic coordinate system is represented by a graticule of intersecting parallels (latitude) and meridians (longitude).

1) A geographic coordinate system is represented by a graticule of intersecting parallels (latitude) and meridians (longitude). 2) A geographic coordinate system uses a three-dimensional spherical model to identify specific locations on the earth. 3) A geographic coordinate system's coordinates are based on latitude and longitude values.

Use the following guidelines for choosing a projection type based on the shape of the geography being mapped:

1) For map areas that extend north-south, use a cylindrical projection. 2) For map areas that extend east-west, use a conic projection. 3) For map areas that have equal extent in all directions, use an azimuthal projection.

Give 2 examples of elliptical deviations on Earth

1) For one thing, the south pole, the most southerly point on the minor axis, is closer to the major axis (the equator) than is the north pole, the most northerly point on the minor axis. 2) Also, the earth's spheroid deviates slightly for different regions of the earth

At what scale do differences in spheroids generally become visible?

1:5,000,000

How many coordinates does a location have defined to it in GIS

2-3

Components of a geographic coordinate system: A datum

A datum provides a frame of reference for measuring locations on the surface of the earth. It defines the origin and orientation of latitude and longitude lines. While a spheroid approximates the shape of the earth, a datum defines the position of the spheroid relative to the center of the earth. The underlying datum and spheroid to which coordinates for a dataset are referenced can change the coordinate values.

Geographic coordinate system

A geographic coordinate system (GCS) uses a three-dimensional spherical model to identify points or areas on the surface of the earth. The pairs of coordinate values that identify a feature on a map are relative to its geographic coordinate system. Each coordinate system is commonly illustrated with a network of intersecting lines of latitude (parallels) and longitude (meridians) called the graticule.

Graticule

A graticule is imaginary on the earth, but it is drawn on globes and maps for reference. Combining the latitude and longitude lines creates a graticule

Components of a geographic coordinate system: Prime meridian

A prime meridian is the zero value for longitude. While the official prime meridian passes through Greenwich in southeast London, United Kingdom, the choice of prime meridian for a given coordinate system is somewhat arbitrary. It is not necessary to define a zero value for latitude because it is always the equator.

Projected coordinate system

A projected coordinate system is based on a geographic coordinate system. Projected coordinate systems are used to convert feature locations from the spherical earth to a flat map. To do so, latitude and longitude coordinates from geographic coordinate systems are projected to planar coordinates

Components of a geographic coordinate system: Spheroid

A spheroid is the mathematical model that estimates the size and shape of the earth. Because the earth's surface is not perfectly symmetrical, the semimajor and semiminor axes that fit one geographical region do not necessarily fit another one, which is one reason why there are multiple spheroids.

What is the shape of the Earth

An Ellipsoid (spheroid) is based on an ellipse

x-coordinate y-coordinate z-coordinate

An x-coordinate represents the horizontal distance from the point of origin; a y-coordinate represents the vertical distance from the point of origin; and a z-coordinate represents the height of the point from the ground (for use in 3D visualizations).

To create a map for measuring how much total land is part of a national park, which spatial property should be preserved?

Area

Map distortions: Azimuthal projections

Azimuthal projections preserve direction from one or two points to every other point. For this example, the azimuthal equidistant projection, distances and directions to all places are true only from the center point of the projection. Any distance or direction measured from the center of the map will be accurate, but any distance or direction measured from any other point will be inaccurate.

Projection Types Azimuthal projections (planar projections)

Azimuthal projections, also called planar projections, project map data onto a flat surface. When that point is either the north or south poles, longitude lines radiate outward from the pole at their true angle. Latitude lines appear as a series of concentric circles. Azimuthal projections are used most often to map the polar regions.

To create a map that has minimal distortion but does not perfectly preserve any of the four spatial properties, which type of projection should be used?

Compromise

Map distortions Compromise projections

Compromise projections try to balance shape and area distortion. No flat map can be both equal area and conformal; you need a globe for that instance. One widely used example is the Winkel Tripel projection, which minimizes overall distortion but does not preserve any of the four spatial properties. Compromise projections are named because no one property is completely accurate, but no property is extremely inaccurate.

Projection Types: Cylindrical projections

Conceptually, cylindrical projections are created by wrapping a cylinder around a globe and projecting light through the globe onto the cylinder. Cylindrical projections represent meridians as straight, evenly spaced, vertical lines; they represent parallels as straight, horizontal lines. Meridians and parallels intersect at right angles, as they do on the globe.

Map distortions: Conformal projections

Conformal projections preserve shape but not area. In this example, the Mercator projection is conformal in that angles and shapes within any small area are fairly accurately depicted. All the continents are the right shape, but Greenland looks disproportionally large, and Africa looks small.

Projection Types: Conic projections

Conic projections are created by setting a cone over a globe and projecting light from the center of the globe onto the cone. The simplest conic projection contacts the globe along a single latitude line called the standard parallel. In general, distortion increases north and south of the standard parallel.

Do maps distort?

Converting a round map to a flat surface results in distortion to the properties of the map. Every projection type causes distortion in a minimum of two map properties (shape, area, distance, or direction)

Which of the following surfaces are developable surfaces for creating map projections? Choose three 1) Cylinder 2) Sphere 3) Ellipse 4) Cone 5) Plane

Cylinder Cone Plane

What does each coordinate

Each coordinate represents a distance from a defined point of origin

How is each coordinate system commonly illustrated?

Each coordinate system is commonly illustrated with a network of intersecting lines of latitude (parallels) and longitude (meridians) called a graticule.

Map distortions: Equal-area projections

Equal-area projections preserve area but not shape. This example, the sinusoidal projection, is an easily plotted equal-area projection for world maps. Here, Greenland looks like the right size in comparison to other land masses, but North America and Australia are the wrong shape.

Map distortions: Equidistant projections

Equidistant projections preserve distance from one or two points to every other point. In this example, showing the equidistant conic projection, distances are true only along all meridians and along one or two standard parallels. A distance measured from the north pole along one of the latitude lines will be accurate, but a distance measured along one of the latitude lines will be distorted.

If you cannot find the original projection for geographic data, you can use any geographic coordinate system that is appropriate for the region of the world that you are mapping. True-False

False

No matter which coordinate system that you use, a specific location on the earth will have the same coordinates

False

When a more accurate spheroid has been produced, all spatial geodatabases are updated to use the new spheroid. True- False

False

Map distortions: Gnomonic projections

Gnomonic projections preserve the shortest route (distance and direction) but cannot preserve area. With this example, the north pole gnomonic projection, any straight line drawn on the map is on a great circle, but directions are true only from the center point of projection.

What would happen if you ignored deviations

Ignoring deviations and using the same spheroid for all locations on the earth could lead to errors of several meters, or hundreds of meters in extreme cases, in measurements on a regional scale.

How are longitude and latitude measures?

Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. The angles often are measured in degrees (or in gradians).

How are distances measured on a flat map different to a globe

On a globe, distances are measured in angular degrees (latitude(y-axis) and longitude (x-axis)); on a flat map, distances may be in one of many linear units of measurement.

Projection Types

Projection types result from projecting coordinates from a spherical coordinate system onto a flat surface. These surfaces are used by cartographers as the basis for maps.

Why are new spheriods used on local scales

Technology improves, more spheroids of higher local accuracy will be developed. Remember that changing spheroids changes the location values for the features that you are mapping. Because of the complexity of changing spheroids, ground-measured spheroids will remain in use for several years.

Which of the following statements best describes the shape of the earth?

The earth is an oblate ellipsoid, which closely approximates a sphere

Components of a geographic coordinate system: Angular units of measure

The graticule of latitude and longitude lines is an angular measurement system. All features on the earth's surface are located using measurements that are relative to the center of the earth.

A geographic coordinate system identifies location on a globe using angular degrees rather than linear measurements True- False

True

Coordinate information can be stored as values in a table. True-False

True

Projections cause distortion in a minimum of two of the following spatial properties: area, direction, distance, or shape. True-False

True

How to make sure geographic information is placed accurately on a map

coordinate system

Is the Earth's surface symmetrical

earth's surface is not perfectly symmetrical, so the semimajor and semiminor axes that fit one geographical region do not necessarily fit another one.

what are the two types of coordinate systems used in a GIS

geographic coordinate systems and projected coordinate systems

To create a map to compare global data, which type of spatial reference should be used?

local datum

What spatial properties are cartographers trying to maintain?

spatial property, or properties, that they preserve: shape, area, distance, and direction (azimuth).