Projections Quick Reference

This topic provides a summary of key coordinate information for experienced GIS users.

 

If you are a new user and are working within Manifold System only, jump to the Projections topic and the Projecting a Map topic. Very little expertise in the inner workings of projections is required to make simple projections when working within Manifold System.

 

If you must deal with maps in legacy formats such as ESRI .shp or AutoCAD .dxf, please read the Projections and Legacy Formats topic. Because legacy formats do not normally save information on projection parameters used, importing maps saved in legacy formats can require extensive knowledge of projections and coordinate systems.

 

If you are new to projections and coordinate systems in GIS or would like a refresher on coordinate systems and projections consult the Projections Tutorial and subsequent topics followed by a quick return to this topic as a summary.

 

·      Manifold maintains all coordinates using double-precision, floating-point numbers. This allows mapping the Earth with accuracy to 16 decimal digits of accuracy. If we divide a meter (about a yard) into two hundred and fifty million parts, Manifold's accuracy is better.

 

·      A map shows any combination of drawings or images reprojected "on the fly" into whatever projection is used in that map. Use Edit - Assign Projection to specify the projection a map uses. This changes the projection used by the map without changing the native projections of any drawing or image used in the map. Because a map window allows multiple layers it is the preferred window for work in Manifold.

 

·      Drawings and images are shown in drawing windows and image windows using their native coordinate systems. Drawings and images may be reprojected so their internal coordinates are already transformed into a specified projection. If a particular projection will be frequently used with drawings or images in a map it is wise to convert them into that projection so there is no need to reproject them on the fly.

 

·      Use Edit - Change Projection to transform the coordinate numbers inside a drawing or image into their equivalents for the desired new projection. This is a permanent alteration in the internal coordinates used to create a drawing or image. This dialog allows export of projection information to XML, Golden Software GSR or ESRI PRJ formats (as well as import from those formats) to allow for easy creation of accessory files when exporting components for use in other software package. The dialog also allows quick fetching and reuse of coordinate information specified for other components, to avoid having to manually re-enter coordinate information whenever possible.

 

·      Images that originate in raster data are normally not re-projected since doing so invariably adds or removes pixels via interpolation. Instead, raster data images are re-projected on the fly in Map view to achieve the desired effect. In contrast, images used purely for visual effects are often re-projected using the Edit - Change Projection dialog so maps that use them will display rapidly.

 

·      CAD-style drawings created using linear coordinates such as meters or inches are interpreted by default as using the Orthographic projection centered on the 0,0 origin of the Prime Meridian and the Equator. If not otherwise georeferenced, images also are placed with their lower left-hand corner at that geographic 0,0 origin and are taken to be in Orthographic projection. Manifold's internal accuracy is so high that images scanned at over 4800 DPI can have a distinct geographic location for each pixel.

 

·      Since images shot from aircraft or satellites are often overhead views of very small portions of the Earth, they may be conveniently used within the default Orthographic projection used for images. This allows them to be used in a casual way in map layers with cartographic drawings that also use Orthographic projection centered on the current view. This is a fast setup where Map view can operate without any re-projection on the fly. We can use the various image georegistration tools to adjust such images so they are a good match to the vector drawings.

 

·      All drawings using projections other than degree-based "Latitude / Longitude" are based on linear measures, usually meters, centimeters or feet as units of measure. When importing from a "smart" format the coordinates properties for such drawings will be automatically set by Manifold during import.

 

·      Legacy formats like ESRI "shape files" are not normally used for saving maps in projected form. If we have the misfortune of encountering a projected map saved in a legacy format we must make all efforts to locate and grab any documentation that accompanies the file to see what projection parameters were used. We will need this information to tell Manifold what to do with the numbers being imported. After such maps are imported into Manifold as drawings, we must manually change the coordinates properties for that drawing using the Edit - Assign Projection dialog to specify the correct projection parameters based on what we read in the metadata. Note that even if the shapefile is in unprojected Latitude / Longitude coordinates we will have to specify which datum is used using this dialog.

 

·      The Edit - Assign Projection dialog is normally used only once when importing projected drawings from legacy GIS formats. Requisite parameters may be manually added with this dialog after importing the drawing. Changing the projection properties of a drawing does not change the raw coordinate numbers that define the drawing's objects. It simply changes the way the numbers are interpreted by the system.

 

·      It's often the case that images used in mapping will not be a perfect overhead view of the subject. We will often receive such images in geographically-mute image formats such as .jpeg that have nothing to say about the projection used. Technically, such images are often in Tilted Perspective or Space Oblique projection but with unknown parameters. In Manifold, we deal with such images by assuming they are Orthographic and then we use Manifold georegistration tools to warp the image to fit a "known good" vector drawing or set of lat/lon coordinates. In effect, we are re-projecting the image into Orthographic by nudging its scale and position while warping it. This georegistration process is usually easy and very satisfactory in the case of near-overhead images.

 

Very Important

 

To re-project a component, use the Edit - Change Projection dialog. Do NOT use the Edit - Assign Projection dialog. This latter dialog is a specialized dialog that is used to manually correct the projection assigned to a component imported from a format that is unable to provide projection information.

 

Customization

 

Experts may specify custom units of measure, ellipsoids, datums and even customized coordinate systems (projections) by specifying customized projection presets. See the Customization topic.