# A gis represents reality, it is not reality a gis represents reality, it is not reality

 Sana 10.06.2019 Hajmi 1.27 Mb. #64238

• ## To determine location of features in real world or on map need a reference system

• A set of lines of known location that can be used to determine the locations of features that fall between the lines

• ## Coordinate Systems

• Reference systems used to determine feature locations
• ## In this module learn about

• different coordinate systems
• how they work
• how to change the coordinate system of a map.

• ## Two types of coordinate systems

• Geographic
• Used to locate objects on the curved surface of the earth
• Projected
• Used to locate objects on a flat surface
• a paper map or a digital GIS map displayed on a flat computer screen.
• ## Each attempts to model earth and feature locations accurately

• But no system is completely accurate

• ## Consists of a network of intersecting lines called a graticule

• Intersecting lines = longitude and latitude

• ## Graticule

• Longitude
• Vertical lines
• Latitude
• Horizontal lines

• ## Measurements expressed in

• Degrees
• 1/360th of a circle.
• Can be divided into 60 minutes
• Minutes
• Can be divided into 60 seconds
• Seconds

• ## Lines of longitude

• Called meridians
• Measures of longitude begin at the prime meridian
• Defines zero value for longitude
• Range from 0° to 180° going east
• Range from 0° to -180° going west

• ## Prime meridian

• Green line
• Starting point for longitude
• Has a value of 0
• ## Equator

• Red line
• Starting point for latitude
• Has a value of 0
• Runs midway between the north and south poles
• Dividing earth into northern and southern hemispheres.

• ## For example, consider these coordinates:

• Longitude: 60 degrees East (60° 00' 00")
• Latitude: 55 degrees, 30 minutes North (55° 30' 00")
• ## Longitude coordinate refers to angle formed by two lines

• one at the prime meridian
• the other extending east along the equator.
• ## Latitude coordinate refers to angle formed by two lines

• one on the equator
• the other extending north along the 60° meridian.

• ## Many models of the earth's shape

• Each has its own geographic coordinate system
• ## All based on

• degrees of latitude and longitude

• Sphere
• Spheroid

• ## Assuming the earth is a sphere greatly simplifies mathematical calculations

• Works well for small-scale maps
• Maps that show large area of the earth
• ## A sphere does not provide enough accuracy for large-scale maps

• maps that show smaller area of earth in more detail

• ## Planet Earth

• slightly pear-shaped and bumpy
• has several dents and undulations
• south pole is closer to the equator than north pole
• ## Geoid

• Model for complicated of earth
• Too mathematically complicated to use for practical purposes, so spheroid is used as a compromise

• ## Some spheroids were developed to

• Model the entire earth
• Model specific regions more accurately
• ## World Geodetic System of 1972 (WGS72) and 1984 (WGS84)

• Used to represent the whole world
• ## Clarke 1866 and Geodetic Reference System of 1980 (GRS80)

• Most commonly used in North America

• ## Why do you need to know about spheroids?

• Because ignoring deviations and using the same spheroid for all locations on the earth could lead to measurement errors of several meters or, in extreme cases, hundreds of meters.

• ## For this purpose, a geographic coordinate system uses a datum.

• A datum specifies which spheroid you are using as your earth model and at which exact location (a single point) you are aligning that spheroid to the earth's surface.

• ## Red spheroid

• Aligned to the earth to preserve accurate measurements for North America
• ## Blue spheroid

• Aligned to the earth to preserve accurate measurements for Europe

• ## Datum

• Defines origin of geographic coordinate system
• The point where the spheroid matches up perfectly with the surface of the earth and where the latitude-longitude coordinates on the spheroid are true and accurate.
• All other points in the system are referenced to the origin.
• In this way, a datum determines how your geographic coordinate system assigns latitude-longitude values to feature locations.

• ## For example, consider a location in Redlands, California, that is based on the North American Datum of 1983

• The coordinate values of this location are:
• –117° 12' 57.75961" (longitude) 34° 01' 43.77884" (latitude)
• ## Now consider the same point on the North American Datum of 1927

• –117° 12' 54.61539" (longitude) 34° 01' 43.72995" (latitude)

• ## In both NAD 1927 and the NAD 1983 datums

• Spheroid matches the earth closely in North America
• Is quite a bit off in other areas

• ## NAD 1927

• origin aligns the Clark 1866 spheroid with a point in North America
• ## NAD 1983

• Origin aligns the center of the spheroid with the center of the earth

• ## The most recently developed and widely used datum for locational measurement worldwide is

• World Geodetic System of 1984 (WGS 1984)

• ## To convert feature locations from the spherical earth to a flat map

• Latitude and longitude coordinates from a geographic coordinate system must be converted, or projected, to planar coordinates

• ## Projected coordinate system

• A reference system for identifying locations and measuring features on a flat (map) surface
• Consists of lines that intersect at right angles, forming a grid
• Based on Cartesian coordinates
• Have an origin, an x and a y axis, and a unit for measuring distance

• ## The origin of the projected coordinate system

• (0,0)
• commonly coincides with the center of the map.
• ## This means that x and y coordinate values will be positive only in one quadrant of the map (the upper right).

• On published maps, however, it is desirable to have all the coordinate values be positive numbers.

• ## To offset this problem

• Mapmakers add 2 numbers to each x and y value
• Numbers are big enough to ensure that all coordinate values, at least in the area of interest, are positive values.
• False easting
• Number added to the x coordinate
• False northing
• Number added to the y coordinate

• ## ArcMap can still project the data on the fly, but it can no longer guarantee perfect alignment.

• For perfect data alignment, you need to apply a transformation to make the geographic coordinate systems match

• ## How do you know what coordinate system your data is stored in?

• You can view the coordinate system information for a dataset in ArcCatalog™, in its metadata.
• If a dataset has no coordinate system information in its metadata (it's missing), you may not be able to display the data in ArcMap.
• You may need to do some research to find out the coordinate system, then define the coordinate system using the ArcGIS tools provided.

• ## Map units

• Units in which coordinates for a dataset are stored
• Determined by the coordinate system
• If data is stored in a geographic coordinate system
• Map units are usually decimal degrees
• If data is stored in a projected coordinate system
• Map units are usually meters or feet
• Units can be changed only by changing the data's coordinate system.
• ## Display unit

• Independent of map units
• Are a property of a data frame
• The units in which ArcMap displays coordinate values and reports measurements.
• You can set the display units for any data frame and change them at any time.

• ## Degrees can be expressed two ways:

• degrees, minutes, seconds (DMS)
• decimal degrees (DD).

• ## Map projection

• Used to convert data from a geographic coordinate system to a projected (planar) coordinate system
• ## There are many different map projections

• Each preserves the spatial properties of data (shape, area, distance, and direction) differently

• ## Maps are always flat, so do you always need a map projection?

• Maybe—it depends on what you want to do.

• ## Use a map projection to convert data to a projected coordinate system

• If you need to perform analysis
• measure distances, calculate areas and perimeters, determine the shortest route between two points
• If you need to show a particular spatial property for features on a map as it really exists on the earth

• Cylinder
• Cone
• Plane

• ## Cylinder

• wrapped around the earth so that it touches the equator
• accurate in the equatorial zone

• ## Plane

• touches the earth at a pole
• accurate in the polar region.

• ## Produce maps with

• straight, evenly-spaced meridians
• straight parallels that intersect meridians at right angles
• ## Created by

• wrapping a cylinder around a globe
• projecting a light source through the globe onto the cylinder
• cutting along a line of longitude
• Being laid flat

• ## Produce maps with

• straight converging longitude lines
• concentric circular arcs for latitude lines
• ## Created by

• setting a cone over a globe
• projecting light from the center of the globe onto the cone
• cutting along a longitude line

• ## Produce maps on which

• longitude lines converge at the north pole and radiate outward
• Latitude lines appear as a series of concentric circles
• ## Created by

• passing a light source through the earth onto a flat surface (plane).
• In this example, the plane touches the earth at the north pole.

• Shape
• Area
• Distance
• Direction

• ## Shape

• Shapes, such as outlines of countries, look the same on the map as they do on the earth.
• Called "conformal”
• Compass directions are true for a limited distance around any given location

• ## Area

• Size of a feature on the map is the same relative to its size on the earth
• If you draw a shape and move it around the map, no matter where you place it, its size will be the same

• ## Distance

• A line between one point on the map and another is the same distance as it is on the earth (taking scale into consideration).
• Most maps have one or two lines of true scale.
• An equidistant map preserves true scale for all straight lines passing through a single specified location
• i.e. if the map is centered on Moscow, a linear measurement from Moscow to any other point on the map would be correct

• ## Direction

• Direction, or azimuth, is measured in degrees of angle from north
• preserves direction for all straight lines passing through a single, specified location
• Directions from one central location to all other points on the map will be shown correctly

• ## When choosing a map projection, think about which properties you want to preserve.

• If your map is large-scale (shows a relatively small area of the earth), the effect of a map projection will be much less than if your map is small-scale (shows a large portion of the earth's surface).

• ## Which map projection you choose for a particular map depends on

• Map's purpose
• Spatial properties you want to preserve

• ## If map will be used for general reference or in an atlas

• Want to balance shape and area distortion.
• In this case, a compromise projection such as the Robinson projection may be the best choice.
• ## If map has a specific purpose

• May need to use a projection that preserves a specific spatial property

• ## Other factors to consider when choosing a map projection

• the size of the area you're mapping,
• the orientation (east-west or north-south)
• the particular portion of the earth that is covered.
• ## When working at a large scale

• distortion doesn't play a big role
• almost any projection centered on your area will be appropriate
• ## In some situations, decision of which map projection to use has already been made

• State Plane and UTM are standard for mapping U.S. states