Autodesk 15606-011408-9320 User Guide - Page 112

layers should fall within which of these three display ranges and set display ranges for them accordingly. For more information, see "Creating a Design Spreadsheet" on page 114. Designing the Map After you have planned the data and layers for your map, you need to address design issues, such as which coordinate system to use, what coordinate precision you want, and how to set up and use a design spreadsheet-as well as following general design guidelines. About Coordinate Systems A coordinate system is a description of the origin of your map, the projection system used, and the units you are using; it describes how the sphere of the earth is projected onto a sheet of paper and converted to the Cartesian coordinate system. Mapping applications use coordinate systems to determine where to display features on the map. In a latitude/longitude coordinate system, each coordinate pair corresponds to an exact fixed location on the earth; in an arbitrary xy coordinate system, the coordinates have meaning in relation to each other but do not necessarily correspond to exact fixed locations on the earth. For example, a floor plan map might specify that the southwest corner is 0,0 and the northeast corner is 500,500, so that if the building were moved to another location, these coordinates would still be accurate because they are not tied to exact locations on the earth. If you used latitude/longitude coordinates for that map, however, the coordinates would change if you moved the building, because each coordinate pair corresponds to one exact point on the earth. Some types of arbitrary xy coordinate systems, such as the state plane coordinate systems, make use of both of these concepts. Each coordinate system represents a specific area, usually a state or a section of a state. Within that area, the coordinates are meaningful and represent exact locations on the earth. So a point with the coordinates 100000,100000 will always represent a specific location in the New York state plane coordinate system. However, if you change the coordinate system to another state plane coordinate system, all of the data will move to that area, where 100000,100000 represents a specific location in that area. In this way, the data is arbitrary xy, because the numbers themselves do not represent exact locations except within the context of the fixed areas. 112 | Chapter 6 Planning and Designing Maps