A scale factor is the ratio of the true map scale to the stated map scale for a particular location.

Two-point equidistant: two "control points" are arbitrarily chosen by the map maker. To avoid negative numbers for locations south of the Equator, NIMA's cartographers assigned the Equator an arbitrary false northing value of 10,000,000 meters. This pseudocylindrical projection is used primarily as a novelty map.

Table 23.1. (b) Secant conic projection. Every x-coordinate is now its original value plus 7,000,000. The following table describes the various map projections and their uses. The mapping of meridians to vertical lines can be visualized by imagining a cylinder (of which the axis coincides with the Earth's axis of rotation) wrapped around the Earth and then projecting onto the cylinder, and subsequently unfolding the cylinder. 23.1. It is almost always the center of the projection (gistutorial.blogspot.com/2011/04/projection-parameters.html). Latitude of origin defines the origin of the ycoordinates. Latitude of center is used with the Hotine Oblique Mercator Center (both Two-Point and Azimuth) cases to define the origin of the ycoordinates. This model is not used for mapping due to its complexity but is instead used for control purposes in the construction of geographic datums. [25] An equal-area projection, however, causes distortions in shape and distance. The Survey Department will officially switch to UTM, when the second phase of the GRN exercise is complete all over Ghana. The meridian opposite the cut line becomes the central meridian.

Module 11: Applications of Remote Sensing for Eart Last modified: Friday, 31 January 2014, 8:30 AM, The representation of geographic features depends on the spacing of the parallels. Instructors and contributors: Jim Sloan, Senior Lecturer, John A. Dutton e-Education Institute; Ryan Baxter, Senior Research Assistant, John A. Dutton e-Education Institute, Beth King, Senior Lecturer, John A. Dutton e-Education Institute and Assistant Program Manager for Online Geospatial Education, and Adrienne Goldsberry, Senior Lecturer, John A. Dutton e-Education Institute; College of Earth and Mineral Sciences, The Pennsylvania State University. This applies for the, Scale is constant along all straight lines radiating from two particular geographic locations. Every distinct map projection distorts in a distinct way.

The meridians drawn on the map help the user to realize the shape distortion and mentally compensate for it. 21. But when you set a projection and flatten everything out, you also start using a new way to measure location. The heavy lines are the x- and y-axes, which divide the grid into four quadrants.

A scale factor of 0.5 means that distance measurements are twice too short. Fig. Was a scale factor used?

(ESRI Map projections.pdf), Fig. The oblique projection is used to align an area on a map so that the map has north oriented in any direction on the map other than 0, 90, 180, or 270 degrees. 23.3. A projection of a global map onto the surface of a polyhedron, which, when expanded to a flat, two-dimensional map, retains most of the relative proportional integrity (relative size and shape) of global features. (. The Pennsylvania State University 2020. Unlike the Orthographic projection, this perspective projection views the globe from a finite distance. It is a coordinate system that divides the 50 states of the United States, Puerto Rico, and the U.S. Virgin Islands into more than 120 numbered sections, referred to as zones.

Historically, datums have been based on ellipsoids that best represent the geoid within the region the datum is intended to map. It is most suited for large scale mapping of areas predominantly north-south in extent. Rhumb lines, lines of constant bearing, but not most great circles, are straight lines on a Mercator projection. Database | Annexes | Site Map, 5.4 Communication with other GIS programs, A2. Some map projections are clearly better for specific purposes than others, so the purpose of a map must be considered to determine which projection should be used. As the name implies, all circular parallels are spaced evenly along the meridians. This projection was developed to provide a conformal map of Alaska with less scale distortion than other conformal projections. These surfaces are cylindrical (e.g. (www.geo.hunter.cuny.edu//Projection%20parameters.htm). A globe is the only way to represent the earth with constant scale throughout the entire map in all directions. The poles are the "normal aspect" (the viewpoint or perspective) which results in the simplest projected grid for this family of projections. B. Garo, 1997, Introduction to Map Projections). (Source:www.geo.hunter.cuny.edu//Projection%20parameters.htm).

This azimuthal projection uses the center of the earth as its perspective point. The azimuthal conformal projection, also known as the. Classification for land use mapping and planning, Level of expertise required for this Chapter: Advanced. Like the latitude of origin, they define the starting point of the y-coordinates; unlike it, they are nearly always the middle parallel of the projection. Presentation | 7. Scale factor is a unitless value applied to the centerpoint or line of a map projection.

stereographic). This gives the illusion of a three-dimensional globe. The, For more complex cylindrical projections the cylinder is rotated, thus changing the tangent or secant lines. One specifies its horizontal position and the other its vertical position. These lines of true scale are called central meridians for cylindrical projections, and standard parallels for conic projections. In particular, Conic projections use this parameter to set the origin of the ycoordinates below the area of interest. Top: The Cylindrical Equal Area projection has a single standard parallel. MIT Information Systems & Technology website, Fundamentals of a shapefiles's coordinate system, Specifying a coordinate system (in the data frame). This family of projections are typically used to represent the entire world. (Laurie A.

Thus, a point of tangency inside the globe results in a line of tangency on the surface (secant planar projections), and a line of tangency inside the globe results in two lines of tangency on the surface (secant conic or secant cylindrical projections)[36].

The central meridian is straight and the 90th meridians are circular arcs.

The point of contact may be the North Pole, the South Pole, a point on the equator, or any point in between. Note 2 In that instance, you don't need to set a false northing parameter to ensure that all ycoordinates are positive. Choose the Project tool under either the Raster or Feature menu (depending on whether your data is raster or vector), still within the Projections and Transformations toolset. Cylindrical projections map the whole Earth as a finite rectangle, except in the first two cases, where the rectangle stretches infinitely tall while retaining constant width. Originally created to display accurate compass bearings for sea travel. Each remaining case has a pair of identical latitudes of opposite sign (or else the equator) at which the east-west scale matches the north-south-scale. This projection is nearly conformal and has little scale distortion within the sensing range of an orbiting mapping satellite such as Landsat. Great circles passing through the focus are represented by straight lines; thus the shortest distance from the center to any other point on the map is a straight line. This projection is neither equal area nor conformal. Great circles are displayed as straight lines: Direction to a fixed location B (the bearing at the starting location A of the shortest route) corresponds to the direction on the map from A to B: Compromise projections give up the idea of perfectly preserving metric properties, seeking instead to strike a balance between distortions, or to simply make things "look right". Azimuthal projections, the spherical (globe) grid is projected onto a flat plane, thus it is also called a plane projection. Background to cartography and raster images, A6. B. Garo, 1997, Introduction to Map Projections). The following projections preserve area: An equidistant map shows distances correctly. A third model of the shape of the Earth is called a geoid, which is a complex and more or less accurate representation of the global mean sea level surface that is obtained through a combination of terrestrial and satellite gravity measurements. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. 23.6. The name of this projection translates into many cones and refers to the projection methodology; used for most of the earlier USGS topographic quadrangles. Equal-area projections are well-suited for maps of general interest and for those showing distributions over space, such as population, wildlife habitats, and land cover. Thank you for your feedback.

Learn more about the Transverse Mercator projection (gistutorial.blogspot.com/2011/04/projection-parameters.html). Choose a coordinate system from the tree or click the Import button and browse to a data source that is defined with the coordinate system you want to use. Possible aspects are polar, equatorial, and oblique.

The Gall Stereographic cylindrical projection results from projecting the earth's surface from the equator onto a secant cylinder intersected by the globe at 45 N and 45 S. This projection moderately distorts distance, shape, direction, and area. Azimuthal projections are classified in part by the focus and, if applicable, by the perspective point. Mercator), conic (e.g., Albers), or azimuthal or plane (e.g. Scale factor is a unitless value applied to the centerpoint or line of a map projection. The point of view determines how the spherical data is projected onto the flat surface. This projection preserves the area of individual polygons while simultaneously maintaining true directions from the center. Fig. The result is a conformal projection that does not maintain true directions. The orientation of the projection surface can be normal (such that the surface's axis of symmetry coincides with the Earth's axis), transverse (at right angles to the Earth's axis) or oblique (any angle in between). This perspective gives the overall effect of the view from a satellite. Kennedy Melita, 1994, Understanding Map projections, Environmental Systems Research Institute, Inc., pp 15 -19. http://rnlnx635.er.usgs.gov/mac/isb/pubs/MapProjections/projections.html, Laurie A. Retrieved September 23, 2017, from, Map Projections - types and distortion patterns. Fig. The Universal Transverse Mercator coordinate system is a specialized application of the Transverse Mercator projection. The various cylindrical projections are distinguished from each other solely by their north-south stretching (where latitude is given by ): In the first case (Mercator), the east-west scale always equals the north-south scale. 23.8.

Its equal-area makes it useful for raster data representation. The cone is cut along the line of longitude that is opposite the central meridian and flattened into a plane. These projections are called secant conic projections and are defined by two standard parallels. It is a representation of the world map on an ellipse with a 2:1 ratio. Attribute database | 5. Directions from the focus are accurate. When data is unprojected, it doesn't have x, y units. (gistutorial.blogspot.com/2011/04/projection-parameters.html). Finally, on the Albers Equal Area Conic projection, the parallels near the northern and southern edges are closer together than the central parallels, and the projection displays equivalent areas (Kennedy, 1994). Some projection parameters, called linear parameters, use these constant units (or they use ratios, such as 0.5 or 0.9996). Distance from any point on the map to each control point is proportional to surface distance on the earth. sets published by the Survey Department, and on informal). NOTE: In some projections, you will also see parameters called the latitude of center and the central parallel. The cylinder is cut along any meridian to produce the final cylindrical projection. These are nothing but two big numbers that are added to each x- and y-coordinate, respectively. Along a line of true scale, such as the equator in this projection, the scale factor is 1.0.

These are nothing but two big numbers that are added to each x- and y-coordinate, respectively.

Both of these need to be defined for your data to display properly. Selecting a model for a shape of the Earth involves choosing between the advantages and disadvantages of a sphere versus an ellipsoid. Rather, they depend on mathematical formulae that have no direct physical interpretation. A line of true scale is defined as having a scale factor of 1.0. The bottom graphic shows the grid (normally invisible) on which the x, y coordinates are located. Inverse projection formulae transform plane coordinates to geographic. The mapping of radial lines can be visualized by imagining a plane tangent to the Earth, with the central point as tangent point. Latitude of center is used with the Hotine Oblique Mercator Center (both Two-Point and Azimuth) cases to define the origin of the ycoordinates. In ArcGIS, you can put the latitude of origin wherever you want for most conic and transverse cylindrical projections. One of the easiest projections to construct because it forms a grid of equal rectangles. A modified equal-area cylindrical projection presented by German political propagandist Arno Peters as original, but is actually identical to the Gall projection. 23.5. This projection is similar to the Mercator projection except that it portrays the world as a circle with a curved graticule. Rationale | 1. (Source:www.geo.hunter.cuny.edu//Projection%20parameters.htm). Plane coordinates like the eastings and northings in the Universal Transverse Mercator (UTM) and State Plane Coordinates (SPC) systems denote positions in flattened grids. Along this line, the actual map scale is equal to the stated scale (there is no distortion of distance). Remember that no map has true scale everywhere. The creation of a map projection involves three steps: Most map projections are not "projections" in any physical sense. This creates two almost parallel lines approximately 180 kilometers, or about 1, away where the scale is 1.0. This projection is similar to the Orthographic projection in that its perspective is from space. At 60 north or south, the scale factor increases to 2.0 along the parallels.

Polar aspects are the simplest form. This creates two secant, or standard, lines on the reference globe. By the geometry of their construction, cylindrical projections stretch distances east-west. An azimuthal map projection of the world, centered on Africa. In order to do this North on the map would be at either a 45 or 135 degree angle to the top of the map. For this reason, azimuthal projections accommodate circular regions better than rectangular regions. However, this is not attainable over an entire map; it is only correct along certain lines or from a specific point. accompany the data sets you might wish to incorporate in your GIS. On the round surface of the earth, locations are described in terms of latitude and longitude.

This oblique cylindrical projection is provided with two options for the national coordinate systems of Malaysia and Brunei.

Not all projections have standard parallels, but many common ones do. (egsc.usgs.gov/isb/pubs/factsheets/fs07701.html). 23.11. It consists of halving the vertical coordinates of the equatorial aspect of one hemisphere and doubling the values of the meridians from the center (Snyder 1987, p. 182).

(Source: www.arcgis.com). (Source: www.icsm.gov.au). The Gnomonic projection views the surface data from the center of the earth, whereas the Stereographic projection views it from pole to pole. Thus the shape of the map for the whole earth is the region between two symmetric rotated cosine curves. Rather than 1.0, the scale along the central meridian of the projection is 0.9996. source: GeoHunter. If you would like to provide more details, please log in and add a comment below. This point becomes the origin of the x, y coordinates. The geographic coordinate system is not a map projection. This line is called the standard parallel. (Occasionally, it's the only angular parameter you can change. The National Imagery and Mapping Agency (NIMA) (formerly the Defense Mapping Agency) adopted a special grid for military use throughout the world called the Universal Transverse Mercator (UTM) grid. The Bonne method has fallen into disuse, giving way to the transverse Mercator method. Combination of the above: the scale depends on latitude only, not on longitude or direction. In other words North can be not be exactly on the top, bottom, left or right side of the map. False northing is a linear value applied to the origin of the y coordinates.

The Krovak projection is an oblique Lambert conformal conic projection designed for the former Czechoslovakia. The Armadillo projection (also called the Raisz Armadillo projection or Orthoapsidal projection) is a world map projection developed by. If you are a GIS practitioner, you have probably faced the need to superimpose unprojected latitude and longitude data onto projected data, and vice versa. This cannot be done without some distortion. Datum: Main menu > Utilities > Latitude and Longitude utilities > Datum > Create datum > Enter name > Enter location (Ghana) > Select ellipsoid > Enter geocentric shift: dx, dy, dz > Enter either Molondenski or Bursa-Wolff parameters from table above > OK- 3. This transverse cylindrical projection maintains scale along the central meridian and all lines parallel to it. Littrow - the only conformal retroazimuthal projection, Hammer retroazimuthal - also preserves distance from the central point, Selection of a model for the shape of the Earth or planetary body (usually choosing between a, Transformation of geographic coordinates (, Reduction of the scale (it does not matter in what order the second and third steps are performed). The meridians are equally spaced, while the spacing between parallel lines of latitude increases toward the poles. To select a map projection, determine which of the properties is the most important for the project and select the coordinate system or map projection that best preserves that property. This modified planar projection shows the true distance from either of two chosen points to any other point on a map. Examples of projections that are conformal include: A map designed around an equal-area (equivalent) projection preserves the property of area so that any area measured on the map is the same as it is measured on the Earth. Ellipsoid: Main menu > Utilities > Latitude and Longitude utilities > Datum > Create datum > Create ellipsoid > Enter name > Enter semi-axis major > Enter flattening (not minor axis) > OK > OK- 2. This creates two almost parallel lines approximately 180 kilometers, or about 1, away where the scale is 1.0. It is famous because it was used for centuries for marine navigation. Each ellipsoid has a distinct major and minor axis. [9] [10], Pseudoconic projections are similar to conic projections in a way that their parallels are partial concentric circles. Fig. (Laurie A. Click OK. All layers in the data frame will now be displayed with that coordinate system. The UTM coordinate system, which uses the Transverse Mercator projection, has a scale factor of 0.9996. Each position has two values that reference it to that central location. These parameters are used mainly with projections that have single points (rather than lines) of zero distortion, such as the Gnomonic and Orthographic. This pseudocylindrical projection projection is used primarily for world maps. There are also times when working with large scales, such as districts or provinces within countries, that distortion doesn't play a significant role, in which any projection that is centered on the area of interest is acceptable. Some projections are designed to minimize the distortion of one or two of the data's characteristics. 2217 Earth and Engineering Sciences Building, University Park, Pennsylvania 16802 Once the earth's back has been broken with a projection, locations are described in terms of constant units like meters or feet. Top: Projected coordinates are positive or negative, depending on their location. North-south stretching grows with latitude, but less quickly than the east-west stretching: such as the Miller cylindrical projection (secant[4/5]). (To compare, you cannot flatten an orange peel without tearing or warping it.). The projection is equivalent to the polar aspect of the Stereographic projection of the spheroid with specific parameters. Open ArcToolbox and select Data Management Tools > Projections and Transformations > Define Projection. Other meridians are complex curves. The representation of geographic features depends on the spacing of the parallels. [1] Since the Earth is roughly the shape of an oblate spheroid, map projections are necessary for creating maps of the Earth or parts of the Earth that are represented on a plane such as a piece of paper or a computer screen. 23.13. The scale factor is usually slightly less than one. This is why georeferenced plane coordinates are referred to as projected, and geographic coordinates are called unprojected. Choose the appropriate coordinate system. On the Lambert Conic Conformal projection, the central parallels are spaced more closely than the parallels near the border, and small geographic shapes are maintained for both small-scale and large-scale maps. (Source:www.geo.hunter.cuny.edu//Projection%20parameters.htm). Types of Azimuthal projection based on focus and perspective point. It is best used for countries that have a long axis, but not an extreme long axis. Using this notation, the coordinates at the origin are x = 0 and y = 0. (ESRI Map projections.pdf), Fig. In ArcGIS, you can change the central meridian of any projection. (a) Equidistant conic, (b) Lambert Conformal Conic Projection, and (c) Albers Equal Area Conic projection. One can state that a dataset is in Transverse Mercator, but that's not enough information. Gall orthographic (also known as Gall-Peters, or Peters, projection), Equirectangular projection - distances along meridians are conserved, Plate carre - an equirectangular projection centered at the equator, Werner cordiform distances from the North Pole are correct as are the curved distance on parallels. GIS handling | 2. This projection shows loxodromes, or rhumb lines, as straight lines with the correct azimuth and scale from the intersection of the central meridian and the central parallel. In the graphic below, data near the poles is stretched. In a few projections, like the Sinusoidal and the Polyconic, every line of latitude has true scale and is therefore a standard parallel. Central parallel defines the origin of the ycoordinates.

A map cannot achieve that property for any area, no matter how small.

Bottom: The Behrmann projection is the same projection, but with two standard parallels at 30 N and 30 S. These standard parallels define the projection and cannot be changed. The cone and globe meet along two latitude lines. You can also use the false easting and northing parameters to reduce the range of the x or y coordinate values. Rather than 1.0, the scale along the central meridian of the projection is 0.9996. A point of tangency occurs if the globe projects onto a flat plane that contacts the globe at only one point[35]. This page was last modified on 24 September 2018, at 12:39. Perspective points may be the center of the earth, a surface point directly opposite from the focus, or a point external to the globe, as if seen from a satellite or another planet. Parallel lines of latitude are projected onto the cone as rings. Transverse cylindrical projections such as the Transverse Mercator use a meridian as the tangential contact or lines parallel to meridians as lines of secancy. This equal-area projection is primarily used for world maps. The signs of x- and y-coordinates in a projected coordinate system. A projected coordinate system is always based on a geographic coordinate system that is based on a sphere or spheroid. However, distortion is increased in other areas. Home Page | 0. As noted in the introduction, any projection of a sphere (or an ellipsoid) onto a plane will have to distort the image. This courseware module is part of Penn State's College of Earth and Mineral Sciences' OER Initiative. All they do is define where the map's x, y units will originate.

The sphere and ellipsoid are not developable surfaces. A Mercator projection with a stated map scale of 1:100,000,000. Every projection also has a latitude of origin. The Mercator projection is one of the most common cylindrical projections, and the equator is usually its line of tangency.

A scale factor of 2.0 means that distance measurements on the map are twice too longif your scale bar tells you it's a hundred kilometers from A to B, it's really only fifty kilometers. Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. In ArcGIS, you can change the standard parallel for some projections and not for others. Projections by preservation of a metric property, List of map projections used to represent the Earth's surface, Choosing a model for the shape of the Earth. When equally spaced, the projection is equidistant in the northsouth direction but neither conformal nor equal area such as the Equidistant Conic projection. Latitude may have a significant impact on choosing which projection that should be used. Commonly, this is in the middle of the map but it doesn't have to be.