Map projections attempt to solve the fundamental problem that the earth is roughly spherical in shape but maps, by necessity, are flat. Map projections are the systems that are used to represent the three-dimensional surface of the earth on a two-dimensional surface (the map). There are a number of different map projection systems but inevitably they all result in some degree of distortion. The map projection systems that are used vary from country to country. Generally each country has a standard projection system (also called a ) that is based on the map projection that is best suited for the country's position and orientation on the globe. Using a nationally-agreed standard means everyone (architects, town planners, surveyors, road engineers, network planners, etc.) can agree about the locations of proposed new buildings, boundaries, roads, etc. In the UK, for example, the national standard coordinate system used in Ordnance Survey maps is called the British National Grid (BNG). It is based on the Transverse Mercator map projection system and gives the coordinate axes a false origin just south-west of the Scilly Isles (in order to ensure that all coordinates in the UK are positive) and divides the country into 100 km squares that each have a two-letter code. Most drive test systems use the GPS (Global Positioning System) to log the mobile device's position. GPS uses the World Geodetic System 1984 (WGS 84) projection system. This is also how we store all coordinate information internally. When the user displays drive test data in the Map view, the MapInfo MapX component automatically converts the coordinates to match the projection system selected for the map being used, so that the data should be aligned correctly. . These are displayed using lines and polygons rather than pixels. For these maps you can select the projection system to be used by right-clicking in the Map view and choosing . Raster maps are bitmap images (generally in GIF or JPEG format). Once imported into the Map view they typically have a companion . file that specifies the coordinates of the map's corners and the projection system being used. Because they are fixed images, MapX cannot redraw them according to the projection system. For this reason, you are not able to select a different projection system for these maps in the Map view. Problems can arise when location binning is used. Suppose the user wants to compare the binned results with a planning tool that uses a geographical grid of the area based on a raster map that uses the national projection system. If the location bin grid is calculated using a different projection system (say the WGS 84 standard) from that used for the map, the location bins won't be aligned with the planning grid. For example, here is some location-binned drive test data displayed on a raster map, which uses the Swiss National Grid projection system. The location binning grid is defined as 50 meter squares based on the projection system option. (This automatically selects the ED79 UTM Zone projection system that corresponds to the first location encountered in the drive test data. There's more about ED79 UTM in the Notice that the location binning grid is skewed relative to the map's grid. This is because the location binning grid has been calculated using a different projection system from the map (the projection system defines an absolute zero position for the grid). Now let's change the location binning projection system to that used by the map (Swiss National Grid). Here is the data displayed on the same map: Additional confusion often occurs because different map projection systems are based on different measurement systems and therefore different units. The map projection systems fall into two main groups—those that are based on latitude and longitude, whose units are a measurement of the arcs of circles (such as degrees), and those that are based on length, typically measured in meters. Users need to specify some measurements (such as location binning sizes) in the same units as used by their projection system. Because each country tends to use a standard coordinate system, customers who are based in one country and work only with data collected in that country do not normally need to think about map projections once they've got everything set up correctly. However, things are more complicated for users who work with data collected in multiple countries that have different standards. Source.


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Last Modified: November 26, 2015 @ 12:00 am