Problems with Projections

There is no way to troubleshoot problems with projections that does not require fairly sophisticated understanding of what projections are all about. Read the Projections topics before beginning any troubleshooting. Don't skip anything since problems with projections almost always involve a battle with obscure details.

 

I want to combine several maps into one and the data won't line up. Objects in drawings using the same projection do not align well.

 

·      Are you trying to combine maps using different UTM zones into one UTM zone map? That won't work due to limitations of UTM. See the Universal Transverse Mercator (UTM) topic. Get the data in unprojected form and use a different projection for a combined map.

·      The same problem occurs with projected maps in Gauss Kruger, State Plane Coordinate System and many other projection systems. These are systems of projections that are designed to allow mapping of limited, local regions within a fixed system of local projections. They are unsuited for combining data from different geographic regions.

·      Is the data accurate to begin with? If the drawings come from different sources it might not be a projection problem but just a matter of differing accuracy in maps from different sources.

·      Are the datums the same? Using exactly the same projection parameters but different datums will result in slightly different positions that will make a difference in high accuracy applications.

·      Did the data originate in unprojected maps or did you import it from projected maps? If imported from projected maps, it could be that the projection parameters you supplied are not the correct ones intended for use with this data. See Projections and Legacy Formats for discussion of the potential problems. Note that publishers of data in projected form will sometimes specify the wrong information for use with their files. The wrong datum is often a problem.

·    Did you use the Edit - Assign Projection dialog in an attempt to set projections? That's a specialized dialog that should be used only to correct an error during import from a format that does not save projection info. Use the Edit - Change Projection dialog to re-project a drawing, image, surface or labels component.

·      If projected data is imported from an older package, did the software used to create the imported files use the same projection standards as Manifold? Manifold uses high accuracy projection formulae and definitions to USGS standards. There is no guarantee that a legacy package uses the same definitions and high accuracy formulae. Avoid this by using only unprojected data from older packages.

 

Drawings, images or surfaces disappear from a map after projection.

 

·    Did you use the Edit - Assign Projection dialog in an attempt to change projections? That's a specialized dialog that should be used only to specify the correct projection after importing from a format that does not save projection info. Use the Edit - Change Projection dialog to re-project a drawing, image, surface or labels component.

·    Was the component imported from a format that does not save projection information? Formats like .dxf and .shp for drawings and .jpg or .gif in images do not save projection information. The component may look visually correct after import, but if it is not imported with the correct projection and georegistration any subsequent re-projection will cause chaos. After importing from a geographically-unaware format you'll have to manually specify the projection by using the Edit - Assign Projection dialog. See Projections and Legacy Formats .

 

Using a projection causes extreme distortion.

 

·      Did you choose the right projection for your intended usage and region? Some projections cause intense distortion. Not all projections are usable for all types of data. See Projecting a Map.

·      Did you specify reasonable projection parameters for your data set and intended usage? Specifying the wrong center latitude and longitude for a projection, for example, can cause even a well-chosen projection to show a highly distorted view.

·      If you specified projection parameters manually after importing a projected drawing from a "dumb" format, it could be that the wrong projection parameters were provided for use with that data or that you made an error when entering the parameters. Check the projection properties to make sure they were entered correctly.

·      Was the component imported from a format that does not save projection information? Formats like .dxf and .shp for drawings and .jpg or .gif in images do not save projection information. The component may look visually correct after import, but if it is not imported with the correct projection and georegistration any subsequent re-projection will cause chaos. After importing from a geographically-unaware format you'll have to manually specify the projection by using the Edit - Assign Projection dialog. See Projections and Legacy Formats .

·      If you attempt to change a projection by changing the Edit - Assign Projection dialog you are not changing the projection but rather are changing Manifold's interpretation of the existing coordinate values. Change the projection using Edit - Change Projection to permanently change the projection for drawings, images, surfaces and labels. Use Edit - Assign Projection to change the projection used by a map.

 

Objects drawn in maps change shape or position unexpectedly after projection.

 

·      Are large objects involved? When drawing an object in any geographic drawing it is important to use Segmentization to prepare the object for possible re-projection.

·      Is the center of the projection (for projections that have specifiable centers) reasonably near the object? Many projections will greatly distort objects that are not near the center.

·      Have you chosen a sensible projection given the location of the object? For example, using a "whole world" pseudocylindrical projection to show a single island near the Arctic circle is a poor choice if you wish to avoid distortion.

·      When using the Orthographic projection, have you checked the Clip Coordinates box? If not, the entire world is "wrapped" around the projection and objects from the "opposite" side of the world will be seen co-mingled with objects from the "front" side of the world.

 

Changing a projection is not permanent

 

·      Changing a projection in a map with Edit - Assign Projection only changes the projection used by that particular map. It won't change the native projection of components used as layers in the map. If you later open one of those components in its own window or in a different map window the projection will not have been changed. To permanently change the projection of a component used in a map, use the Project to Map choice in the layer tab context menu to re-project it to the map's projection or open the component in its own window and use Edit - Change Projection to specify any projection desired.

 

Re-projecting an image or other large component causes error messages

 

Manifold cannot work correctly if your machine does not have adequate RAM, an operating system that can handle the necessary RAM, or adequate disk space.

 

·      Are you using large images? Many image formats are compressed. When imported, the images they contain will expand to require considerably more RAM than indicated by the size of the file on disk.

·      Are you using Windows XP or 2000? If not and you are working with larger projects you have probably encountered a bug in Windows, not in Manifold.

·      Do you have enough free space on disk? Larger projects should have gigabytes free on disk.

·      Is your page file large enough? Allow 2.5 gigabytes per running instance of Manifold.

·      Are you running other applications at the same time as Manifold? Other applications could be using up RAM that is needed for Manifold.

·      See the Performance Tips topic for additional information.