Week 4 GEOG 360
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Oregon State University, Corvallis *
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Course
360
Subject
Geography
Date
Feb 20, 2024
Type
docx
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8
Uploaded by HighnessLoris1319
Week 4
Earth is an oblate ellipsoid, but its major and minor axes to not very greatly (also considered a spheroid)
Today, the GRS80 spheroid is replacing Clarke 1866 in most geographic databases because it is more accurate. However, just because a spatial database covers North America, you cannot assume that it uses the GRS80 spheroid. Many North American databases have not yet been converted from Clarke 1866.
Measure the differences between spheroids
Click Bookmarks and choose Detail 3.
On the Map tab, in the Inquiry group, click the Measure down arrow and choose the Measure Distance tool
.
Geographic coordinate systems
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.
Projected coordinate systems
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.
CORRECT
1.
No matter which coordinate system you use, a specific location on the earth will have the same coordinates.
False
CORRECT
2.
Which statement best describes the shape of the earth?
The earth is an oblate ellipsoid, which closely approximates a sphere.
A geographic coordinate system identifies location on a globe using angular degrees rather than linear measurements.
True
When a more accurate spheroid has been produced, all spatial geodatabases
are updated to use the new spheroid.
False
Combining the latitude and longitude lines creates a graticule.
Each point is referenced by its longitude and latitude values. 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).
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.
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.
To maintain the data's integrity, it is important to create the feature class or shapefile using the data's native coordinate system.
Next to the Coordinate System field, click the Select Coordinate System button
.
Collapse Layers, and then expand Geographic Coordinate System.
Here, you can see all the available geographic coordinate systems, including local and global datums, organized by what geographic region the geographic coordinate system is best suited to.
Expand Geographic Coordinate System, if necessary, and then expand World.
The Search function allows you to search for specific coordinate systems and
filters out all other results.
Add data without a spatial reference
Now you will add a shapefile that contains nearly identical data, except that it is missing its spatial reference.
In the Catalog pane, expand the AfricaCities folder, if necessary, and then drag the XYafrica_cities_no_proj.shp file onto the map.
The data from the shapefile does not display correctly on the map. You know
that it was created in a coordinate system, but the information that identifies
that coordinate system is missing.
For XYafrica_cities_no_proj, change the symbology to Square 3.
When using a GIS, it is important to have a solid understanding of coordinate
systems
to ensure that your map products accurately show your
area of interest.
Choosing the wrong coordinate system, or using more than one coordinate system,
can result in inaccuracies when calculating measurements on your map like area and perimeter
or can trigger errors when performing spatial analysis.
CORRECT
1.
Coordinate information can be stored as values in a table.
True
CORRECT
2.
Which three statements about geographic coordinate systems are true?
Choose three.
A geographic coordinate system uses a three-dimensional spherical model to
identify specific locations on the earth.
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A geographic coordinate system is represented by a graticule of intersecting parallels (latitude) and meridians (longitude).
A geographic coordinate system's coordinates are based on latitude and longitude values.
CORRECT
3.
To create a map to compare global data, which type of spatial reference should be used?
Earth-centered datum
CORRECT
4.
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.
False
To make a two-dimensional map, features on the earth are
recorded on a three-
dimensional globe
and are then projected onto a flat map.
Coordinate systems that use this process to create 2D maps are called projected coordinate systems.
The process begins with a geographic coordinate system,
which describes the values for latitude and longitude in three-
dimensional space.
The latitude and longitude lines create a grid, sometimes called a graticule.
Next, a reference globe is created
by wrapping the grid around a model of the earth.
The reference globe represents the shape of the earth based on a specific geographic coordinate system.
Geographic features are assigned coordinates based on the geographic coordinate system used by the reference globe.
Finally, using a mathematical formula, features on the reference globe's surface are transposed onto a flat surface.
Coordinates on the globe are transformed into x,y values on the "projected" map.
Therefore, if two projections are based on different coordinate systems,
each projection will have a different physical location for the same set of coordinates.
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.
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.
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.
Use the following guidelines for choosing a projection type based on
the shape of the geography being mapped:
For map areas that extend north–south, use a cylindrical projection.
For map areas that extend east–west, use a conic projection.
For map areas that have equal extent in all directions, use an azimuthal projection.
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.
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.
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 longitude lines will be distorted.
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.
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.
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.
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**If a map preserves two spatial properties, one of them is always direction.**
Define the map's area of interest to help choose a coordinate system
A good projection minimizes distortion in your area of interest. A general guideline is to choose a projection according to the latitude of your area of interest.
To map tropical regions, use a cylindrical projection.
To map middle latitudes, use a conic projection.
To map a polar region, use a planar (azimuthal) projection.
Identify the shape and size of the area of interest
Alaska is a large state with a total area of 591,004 square miles, or 1,530,700 square kilometers. With the Alaska Peninsula and the Aleutian Islands, which stretch to the southwest, Alaska extends mainly east to west.
For map areas that extend north–south, use a cylindrical projection.
For map areas that extend east–west, use a conic projection.
For map areas that have equal extent in all directions, use an
azimuthal projection.
CORRECT
1.
Projections cause distortion in a minimum of two of the following spatial properties: area, direction, distance, or shape.
True
CORRECT
2.
Which two statements about projected coordinate systems are true?
Choose two.
A projected coordinate system gives linear measurements on a planar surface from a predefined starting point.
A projected coordinate system's coordinates are measured in linear units, such as feet or meters.
CORRECT
3.
Which three surfaces are developable surfaces for creating map projections?
Choose three.
Cylinder
Cone
Plane
CORRECT
4.
To create a map for measuring how much total land is part of a national park, which spatial property should be preserved?
Area
CORRECT
5.
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