Projections and Coordinate Systems
Both examples are coordinate systems. The difference is that WGS is a geographic coordinate system, and UTM is a projected coordinate. Projected Coordinate Systems. ▫ A map projection is the systematic transformation of locations on the earth . geometric relationships are known, either through. The decision as to which map projection and coordinate reference system to The Mercator projection, for example, is used where angular relationships are.
This means that when you want to carry out accurate analytical operations, you need to use a map projection that provides the best characteristics for your analyses.
For example, if you need to measure distances on your map, you should try to use a map projection for your data that provides high accuracy for distances. In other words, East will always occur at a 90 degree angle to North. Maintaining correct angular properties can be preserved on a map projection as well. A map projection that retains this property of angular conformity is called a conformal or orthomorphic projection.
These projections are used when the preservation of angular relationships is important. They are commonly used for navigational or meteorological tasks. It is important to remember that maintaining true angles on a map is difficult for large areas and should be attempted only for small portions of the earth. The conformal type of projection results in distortions of areas, meaning that if area measurements are made on the map, they will be incorrect.
The larger the area the less accurate the area measurements will be. Geological Survey uses a conformal projection for many of its topographic maps. The Mercator projection, for example, is used where angular relationships are important, but the relationship of areas are distorted.
Such projections, called equidistant projections, require that the scale of the map is kept constant. A map is equidistant when it correctly represents distances from the centre of the projection to any other place on the map.
Equidistant projections maintain accurate distances from the centre of the projection or along given lines. These projections are used for radio and seismic mapping, and for navigation. The Plate Carree Equidistant Cylindrical projection, for example, is used when accurate distance measurement is important. In practice, general reference and educational maps most often require the use of equal area projections.
As the name implies, these maps are best used when calculations of area are the dominant calculations you will perform. If, for example, you are trying to analyse a particular area in your town to find out whether it is large enough for a new shopping mall, equal area projections are the best choice.
On the one hand, the larger the area you are analysing, the more precise your area measures will be, if you use an equal area projection rather than another type. On the other hand, an equal area projection results in distortions of angular conformity when dealing with large areas. Small areas will be far less prone to having their angles distorted when you use an equal area projection. The Mollweide Equal Area Cylindrical projection, for example, ensures that all mapped areas have the same proportional relationship to the areas on the Earth.
Keep in mind that map projection is a very complex topic. In reality, the choice of which projection to use, will often be made for you. Most countries have commonly used projections and when data is exchanged people will follow the national trend.
- 1. Map Projections
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In general CRS can be divided into projected coordinate reference systems also called Cartesian or rectangular coordinate reference systems and geographic coordinate reference systems. The most popular is called WGS Lines of latitude run parallel to the equator and divide the earth into equally spaced sections from North to South or South to North. The reference line for latitude is the equator and each hemisphere is divided into ninety sections, each representing one degree of latitude.
In the northern hemisphere, degrees of latitude are measured from zero at the equator to ninety at the north pole.Geographic and Projected coordinate in ArcGIS -- Map Projection in ArcGIS -- ArcGIS Tutorial
In the southern hemisphere, degrees of latitude are measured from zero at the equator to ninety degrees at the south pole. Geographic coordinate system with lines of latitude parallel to the equator and lines of longitude with the prime meridian through Greenwich.
Lines of longitude, on the other hand, do not stand up so well to the standard of uniformity. Lines of longitude run perpendicular to the equator and converge at the poles.
The reference line for longitude the prime meridian runs from the North pole to the South pole through Greenwich, England. Subsequent lines of longitude are measured from zero to degrees East or West of the prime meridian. Note that values West of the prime meridian are assigned negative values for use in digital mapping applications. At the equator, and only at the equator, the distance represented by one line of longitude is equal to the distance represented by one degree of latitude.
Using the geographic coordinate system, we have a grid of lines dividing the earth into squares that cover approximately The easiest way to try to transfer the information onto a flat surface is to convert the geographic coordinates into an X and Y coordinate system, where x is longitude and y is latitude.
Coordinates can also be "projected" onto two other flat surfaces, a cylinder or cone, and then unfolded into a map. The grid formed by the latitude and longitude on a map is called the graticule. Orientations of the three shapes can also vary between equatorial standard lines of latitudetransverse standard lines of longitudeand oblique standard line other than latitude or longitude.
In addition, each projection effects the distance, area, and angle relationships of the earth surface as portrayed on the map. Ideally, these factors would be consistent to the relationships on the real earth. Unfortunately, some relationships are always distorted. This point, or focus, may be a pole, the equator, or other oblique point. Normally though, the azimuthal projection is used for polar charts due to distortion at other latitudes.
A cylindrical projection usually places the earth inside a cylinder with the equator tangent or secant to the inside of the cylinder. If the cylinder is placed perpendicular to the axis of the earth, the resulting projection is called a transverse projection. A conic projection works best over mid latitudes for this reason. These types of map projections can change for different parts or regions of the world in order to reduce certain distortions.
Projection Distance, Area, and Shape Equidistant azimuthal projection has the distance to the outside of the map portrayed correctly. 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 grads. The following illustration shows the world as a globe with longitude and latitude values: In the spherical system, horizontal lines, or east—west lines, are lines of equal latitude, or parallels.
Vertical lines, or north—south lines, are lines of equal longitude, or meridians. These lines encompass the globe and form a gridded network called a graticule. The line of latitude midway between the poles is called the equator. It defines the line of zero latitude.
The line of zero longitude is called the prime meridian. The origin of the graticule 0,0 is defined by where the equator and prime meridian intersect. Latitude and longitude values are traditionally measured either in decimal degrees or in degrees, minutes, and seconds DMS.
Longitude values are measured relative to the prime meridian. If the prime meridian is at Greenwich, then Australia, which is south of the equator and east of Greenwich, has positive longitude values and negative latitude values.
Data defined on a geographic coordinate system is displayed as if a degree is a linear unit of measure. A physical location will usually have different coordinate values in different geographic coordinate systems. Geographic datum transformations If two datasets are not referenced to the same geographic coordinate system, you may need to perform a geographic datum transformation.
As with the coordinate systems, there are several hundred predefined geographic transformations that you can access. It is very important to correctly use a geographic transformation if it is required.
When neglected, coordinates can be in the wrong location by up to a few hundred meters. Projected coordinate systems A projected coordinate system PCS is defined on a flat, two-dimensional surface. Map projections Whether you treat the earth as a sphere or a spheroid, you must transform its three-dimensional surface to create a flat map sheet. This mathematical transformation is commonly referred to as a map projection. One easy way to understand how map projections alter spatial properties is to visualize shining a light through the earth onto a surface, called the projection surface.
Imagine the earth's surface is clear with the graticule drawn on it. Wrap a piece of paper around the earth. A light at the center of the earth will cast the shadows of the graticule onto the piece of paper.
Coordinate Reference Systems
You can now unwrap the paper and lay it flat. The shape of the graticule on the flat paper is different from that on the earth. The map projection has distorted the graticule.