Understanding Map Projections and Coordinate Systems
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This document discusses map projections and coordinate systems in GIS. It explains that there are two main types of coordinate systems: geographic and projected. Geographic coordinate systems use spherical coordinates to identify Earth surface features, while projected coordinate systems use planar coordinates to identify features by projecting them onto a flat surface. It also discusses datums, which provide a reference framework for measuring locations on the Earth's surface, as well as how map projections introduce distortions like changes to shape, area, distance and direction when converting from a spherical to planar surface.
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Mehr von Lyndy Worsham, GISP
Understanding Map Projections and Coordinate Systems
- 2. Objectives • Gain basic understanding of map projections and why we need coordinate systems in GIS • Know difference between a geographic coordinate system and a projected coordinate system • Know where to look to check the coordinate system of a shapefile
- 3. Coordinate Systems • The geographic coordinate system uses spherical coordinates to identify features on the Earth’s surface. • A projected coordinate system uses planar coordinates to identify features on the Earth’s surface.
- 4. Actual Shape of Earth
- 5. Datum A frame of reference for measuring locations on the surface of the Earth Actual shape of Earth Local datum WGS84 Datum
- 6. Datum Differences A different datum locates the same point in a different place Ohene Djan Stadium WGS84 OSGB36
- 7. N S W E Geographic Coordinate System: Latitude & Longitude
- 9. Map Projections – Distortions Distortions make geographers S.A.D.D. Shape Area Distance Direction
- 10. Map Projections – Types Generally classified by the spatial attribute they preserve Projection Preserves Example Conformal Angles Mercator Equal-Area Area Gall-Peters Equidistant Distance Plate carrée Compromise “Look” Robinson
- 11. Map Projections – Comparison Mercator Projection Robinson Projection
- 12. Mercator Projection – Africa and Greenland
- 13. Map Projections-True Size of Africa
- 14. Coordinate Systems Summary • A coordinate system is made up of: – A reference framework • Geographic – coordinates are measured from the earth’s center using angles • Planar –coordinates are projected onto a flat surface – Unit of measurement • Geographic – decimal degrees – Example: 1.220763, 36.894136 • Planar – meters or feet – Example: Zone 37: 265670, 9864977 - Datum • Local • Global (WGS84)
- 15. Map Projections-Storing Information Coordinate Reference System (CRS) is stored within the shapefile
- 16. Key Points • There are two main types of coordinate systems: __________ & __________ • A ______ is used as a foundation of the coordinate systems as a way to model the shape of the Earth • A ________________is a mathematical formula used to convert locations from a 3 dimensional surface onto a 2 dimensional surface • The Coordinate System Reference System (CRS) is stored in the ___________ of the shapefile geographic projected datum Map projection metadata
- 17. Questions?
Hinweis der Redaktion
- This presentation focuses on map projections and why we need them to practice GIS. The presentation will be followed by a demonstration on how to chose a map projection in QGIS and how to check the coordinate system of a shapefile.
- Broadly speaking, there are two main types coordinate systems: geographic and projected. We talked about the geographic coordinate system earlier when we learned about GPS. What do you remember about the geographic coordinate system? The geographic coordinate system uses latitude and longitude to identify positions on the Earth’s surface. It uses angles to measure locations and those angles are expressed as lat/long values in three different formats. A projected coordinate system, on the other hand, uses distance units such as feet or meters to identify positions on the Earth’s surface.
- Every map projection and coordinate system begins with a precisely surveyed starting point. The starting point and the network of points that extends from it is called the datum. All coordinate systems, whether they are geographic or projected, have a datum, so let’s begin our discussion about coordinate systems here. We touched on datums in one of the earlier sessions when we talked about GPS and how GPS works. To review a bit, datums are essentially a reference system for measuring locations on the ground. Locations are measured in terms of geographic coordinates (i.e., latitude and longitude) or plane coordinates (e.g. meters or feet). To measure and specify coordinates accurately, one first must define the geometry of the surface itself. To understand what I mean, imagine a soccer ball. Now focus on one point at an intersection of three panels. You could use spherical (e.g., geographic) coordinates to specify the position of that point. But if you deflate the ball, the position of the point in space changes, and so must its coordinates. The absolute position of a point on a surface, then, depends upon the shape of the surface. Datums are what define that geometric relationship between a coordinate system grid and the Earth's surface.
- The OSGB36 (Ordnance Survey Great Britain 1936), a local reference system for the UK, locates the same coordinates 490ft or 150m north of the same coordinates using WGS84 (World Geodetic System 1984), the system used by GPS. Different datums will locate the same point in a different place because the underlying model used to measure positions on the surface of the Earth is different. Here we see a local datum and a global datum locating the Accra Sports Stadium in Ghana in different locations. Both of these points are correct, but because the datum used to measure the stadium are different, the location of the same point is is also different. So if you collect data in OSGB36 but you tell me that you used WGS1984 to collect the data, the point can be off by 300 meters on your map. Oheni jan
- So yesterday we talked about the geographic coordinate system. Can anybody tell me the key components of the geographic coordinate system? Uses angles, origin is at the intersection of prime meridian and equator. N = +; S = -; E=+, W = - The geographic coordinate system uses the WGS 1984 datum as a frame of reference to measure points on the surface of the earth. As you know, the geographic coordinate system uses latitudes and longitudes which are based on angular measurements to fit the nearly-spherical shape of the Earth. Latitude and longitude has 3 formats. Can anyone tell me what those 3 formats are: Degrees, minutes, seconds Degrees and decimal minute Decimal degrees What is the best format to use for capturing data? Decimal degrees
- Map projections are mathematical equations used to transform latitude and longitude coordinates to plane coordinates. The most important reason for using plane coordinate systems is that many numerical properties (e.g., area and distance) are much easier to calculate on a plane than a sphere and most maps are produced on a two dimensional surface. Planar coordinates are most often given in meters or feet. SHOW ORANGE.
- Map projections cannot provide absolutely accurate representations of the spherical earth. Converting features from a 3D surface onto a flat surface always leads to some type of distortion in shape, area, distance, or directions. A key way to remember this is projects make geographers SADD. There are thousands of different projections. And each of these map projections has been calculated to preserve one or more specific properties of the data — shape, area, distance or direction. The decision as to which map projection and coordinate reference system to use, depends on the extent of the area you want to work in, on the analysis you want to do, and often on the availability of data. Because the earth is not actually a flat grid, no projection is a completely accurate representation. There are many different map projections that are designed to preserve either shape, distance, area of land, or a combination of these.
- Map projections can be classified according to what spatial attributes they preserve. Conformal - preserve angles is useful for navigational charts and weather maps. Shape is preserved for small areas, but the shape of a large area such as a continent will be significantly distorted. Equal Area - preserves area. Many thematic maps use an equal area projection. Equidistant - preserves distance from one (or a few) select point(s). No projection can preserve distances from all points to all other points. If you will use your map to find features within a certain distance of other features, you should use an equidistant projection. Compromise - preserves a balance of angle, area, distance, and shape. For example, conformal preserves angles; equal area preserves area; equidistant preserves distance, and a compromise projection tries to preserve all four. The type of projection you need depends on the type of map you are creating. If you want to measure how far it is from Dar Es Saalam to Naroibi, it would be best to chose an equidistant projection. However, if you wanted to compare the land size of the Kenya and Tanzania, your results will be more accurate if you use an equal area projection.
- Here’s an example of two different types of projections: Mercator and Robinson. Notice the difference in land mass size. While shape is essentially preserved, area differs greatly between the Mercator and Robinson projection. The Robinson projection is what’s called a “compromise” projection because it distorts all the properties of shape, area, angles, and distance within an acceptable limit. Compromise projections are best used for global data sets. If you wanted 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. The Mercator projection is commonly used for navigational purposes. In fact, the Mercator projection was developed in the early 1500s as a way to help ships navigate across the open seas. However, the Mercator projection distorts the size of objects as the latitude increases from the Equator to the poles. So, for example, Greenland and Antarctica appear much larger relative to land masses near the equator than they actually are.
- Here we see how the Mercator projection distorts the size of objects as you get closer to the north and south pole. So, for example, Greenland and Antarctica appear much larger relative to land masses near the equator than they actually are.
- In reality, Greenland is much smaller in land size than Africa. In fact, you could put all of the USA and China in Africa!
- In QGIS, the entire coordinate system definition is called a Coordinate reference system (CRS). The CRS of a shapefile is stored in the metadata. We will show you how to look that up in the demonstration.