Datum and Projections

Overview Coordinate Systems Datum Map Projections Map Scale Cartesian Geographic Datum Local Global Map Projections Types & Differences Map Scale Large & small

Coordinate Systems A method of representing points in a space of given dimensions by coordinates People working with GIS are interested in analyzing relationships between real-world objects. GIS only depicts reality, it is not reality. Depicting the real world accurately is crucial if you want accurate analysis results. Georeferencing is the process of establishing a relationship between the data displayed in your GIS software and its real-world location. This is accomplished by using a coordinate system. To get accurate analysis results from your GIS, you need to understand and determine your coordinate system.

Coordinate Systems – Cartesian X, + Y + X, + Y X, - Y + X, - Y X Y 40, 50 Cartesian coordinate system Measures of length and angle are uniform On a flat surface, locations are identified by x,y coordinates on a grid, with the origin at the center of the grid. Each position has two values that reference it to the central location; one specifies its horizontal position and the other its vertical position. These two values are called the x coordinate and the y coordinate.

Coordinate Systems – geographic Measures of length and angle are not uniform Latitude longitude The most common location reference is the geographic coordinate system measured in latitude and longitude. This system can be used to identify point locations anywhere on the Earth’s surface. Longitude and latitude are angles measured from the earth’s centre to a point on the earth’s surface. Longitude is measured East and West, while Latitude is measured North and South. Longitude lines, also called meridians, stretch between the north and south poles. Latitudes lines also called parallels, encircle the globe as parallel rings. Latitude and longitude are traditionally measured in degrees, minutes, and seconds (DMS). Longitude values range from 00 at the Prime Meridians (the meridian that passes through Greenwich, England) to 1800 when traveling east and from 00 to 1800 when traveling west from the Prime Meridian.

A frame of reference for measuring locations on the surface of the Earth Datum There are two types of datum: global and local A global datum has its origin placed at the Earth’s currently known centre of mass and is more accurate overall. A local datum is aligned so that it closely corresponds to the earth’s surface for a particular area and can be more accurate for that particular region. Different datum locates the same point in different places

Datum Differences A different datum locates the same point in a different place Ohene Djan Stadium WGS84 OSGB36 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.

Map Projections Map projections convert curved surfaces to flat surfaces A map projection is a method for converting the Earth’s three-dimensional surface to a map’s two-dimensional surface. Projection is a mathematical process that converts points on the Earth to a points on a flat surface. The selection of a suitable map projection is important if we are going to calculate areas, distances, or direction. Map projections are classified according to the distortion characteristics. No project can preserve shape, area, distance and direction.

Map Projections – Comparison Mercator Robinson Notice the difference in land mass size. While shape is essentially preserved, area differs greatly between the Mercator and Robinson projection.

Map Projections – Various Others The Earth can be represented in many different ways. These less common projects are attempts to accurately flatten the 3 dimensional globe.

Map Projections – Distortions Distortions make geographers S.A.D.D. Shape Area Distance Direction The conversion of geographic locations from 3 dimensions to 2 will cause distortions. The projection process will distort one or more of the spatial properties – shape, area, distance, and/or direction. Anyone who uses maps should know which projections distorts which properties and to what extent. For example: - a Peter’s projection will give you accurate area calculations but inaccurate shape - a Mercator projection maintains true direction, but sacrifices accuracy of area and distance - a Robinson projection is a compromise of all the properties

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 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.

Map Projections – Storing Information Many spatial data store the projection details with the data

Map Scale Map scale determines the size and shape of features 1:500

Summary A coordinate system is a way to reference a location A datum is way to model the shape of the Earth A map projection is way represent the 3 dimensional surface of the Earth in 2 dimensions