Manifold can import drawings, images, terrains and tables from a very wide range of formats. Some formats such as SDTS are very modern formats that are easy to import with little user intervention. Other formats are very primitive formats that are poorly suited for data interchange. Some formats well suited for CAD programs are less well suited for GIS. Primitive formats or those formats not originally designed for GIS can require a lot of user intervention to import into a GIS program like Manifold.
When working with modern formats the import process is normally easy and automatic in all respects. For example, Manifold can easily import drawings from MapInfo mid / mif format in almost all cases without any need for user intervention. In such cases we import the drawing, pop it open, verify the projection by launching Assign Projection and clicking OK and we're done.
When working with archaic GIS formats such as ESRI .shp or with formats optimized for CAD work such as AutoCAD .dxf user intervention often will be required. In that case, we may not get away with just verifying the projection by pushing OK in the Assign Projection dialog. Instead, we will have to actually understand what the projection parameters are supposed to be and to change them to the right parameters if necessary. That requires a lot more thought than simply pressing OK.
The most frequent class of technical support questions when using Manifold arise from confusion over importing projected maps from old GIS formats such as ESRI .shp or non-GIS formats such as AutoCAD .dxf. Because such formats require a lot of user involvement when used with projected maps, they require a much greater degree of user understanding for correct import. That's a drag, because it requires the novice user to learn much more about projections than seems fair for a beginner. On the plus side, we don't have to learn very much in order to take advantage of the very large amount of free GIS data available in legacy formats.
This topic is an overview for reasonably experienced users. If you are completely unfamiliar with projections and coordinates you should also read the Projections book topics, beginning with the Projections topic. If importing shapefiles, see also the Import a Shapefile example.
Projections and Older Formats
Problems with old formats usually arise if the data they contain is intended to represent a projected map. If the data represents an unprojected map we can breathe a sigh of relief. Even very old formats can usually be imported quickly if the data they contain is not projected data. However, when old formats are used to save projected maps we will almost always have to participate in the import process by specifying projection parameters manually using the Assign Projection dialog. This is especially true of non-GIS formats such as .dxf that were designed to save CAD drawings. Regardless of how convenient they are for CAD usage, such formats will require extra steps to import into GIS.
Modern GIS formats are "smart" and automatically save the projection parameters in use together with the data. During import, Manifold will fetch all necessary parameters from such modern formats automatically and will load the coordinates properties for that drawing with the correct parameters necessary to use the data.
CAD formats such as AutoCAD .dxf and older GIS formats such as ESRI .shp, do not save the projection parameters in use. While one is tempted to refer to such formats as "dumb" formats in contrast to the "smart" encoding of projection information within modern GIS formats, we suggest the term "legacy" format. After all, such formats work perfectly well when used as originally designed for their intended purpose (usually a more limited purpose than expected of modern GIS applications) and should not be blamed if someone uses them inappropriately.
Importing data from legacy GIS formats causes so much trouble for beginners in GIS that Manifold tries to protect us from newbie errors by reminding users to verify the projection in use when a new component is first opened if Manifold thinks there is a good chance the component has been imported from a format that does not save projection information. Manifold raises an info bar to ask us to verify the projeciton if the component has been imported in Orthographic projection or if it has been imported in Latitude / Longitude with coordinate locations outside the expected range (+/- 90 latitude and +/- 180 longitude).
The classic error is that someone imports projected data from a legacy format like shapefiles and then when they see it looks OK visually they assume it imported OK as well when in fact the correct projection still needs to be assigned. When an info bar pops up reminding us to verify the projection it is more difficult to skip over that necessary step.
When legacy formats are used to save projected maps they require users to keep track manually of the projections used in the file. This is usually done with a readme.txt file, a "metadata" information text file or other documentation that accompanies the data files and specifies what projection parameters should be used with those files. Users must read the documentation to know what parameters to use. If we download the data files and neglect to fetch the documentation that says how to use them we will find ourselves in trouble. Of course, we will also be in trouble if the authors of the data files neglected to provide any accompanying documentation for us to read.
Because the above is such an obvious hassle most sensible people will refrain from using CAD formats or older GIS formats to save projected data. GIS data in such formats therefore is usually unprojected data that may be imported into Manifold with no special user action required except to specify which datum should be used (as shown in the Import a Shapefile example).
On those occasions when we encounter a projected map that is saved in an older format we will have to specify projection parameters after the import. We can do this using the Assign Projection dialog when we verify the projection to be used.
However, there may be no warning that the data contained in the file is projected and so must be interpreted in a way known only to its author. When acquiring data in older formats we should always keep in mind the possibility that the files contain projected data. If we suspect projected data is involved, we should keep an eye out for documentation that explains what projection parameters are to be used with the file. We may have to search long and hard for this information.
To import from a format that does not save projection information:
1. Import the drawing or image using File - Import. Use default settings.
2. Open the drawing or image.
3. Click verification info bar to use the Edit - Assign Projection dialog to specify the projection information that should be used.
Note that complete information on all projection parameters is required. If a projected drawing is supposed to use Lambert Conformal Conic projection then simply saying that it uses Lambert Conformal Conic projection is not enough: all of the various parameters used within that Lambert Conformal Conic projection (such as datum, latitude / longitude center of the projection, standard and any optional parallels used) must be known as well and specified in the Assign Projection dialog.
Users should be careful to get the best information they can on the projections supposedly used by a given drawing or image. Be skeptical and be thorough: publishers of projected data using legacy formats may not realize that the information they provide is inaccurate or insufficient. For example, if one imports projected files in .shp format that show Wisconsin, it is not enough to say the files are in "Wisconsin State Coordinate System (NAD27)." If the files are in the State Plane Coordinate System using NAD27 we need to know whether they are in the Wisconsin North, Central or South zone.
Another issue is that a projected drawing imported from a legacy format may appear to have imported well at first glance. There is not much visual difference between projections and "unprojected" Latitude / Longitude coordinates in many parts of the Earth when small regions are in view. If we don't use the imported drawing together with other drawings it could take us a while to realize that someone fooled us by providing a projected data set in a legacy format.
It is also quite possible that a Latitude / Longitude unprojected drawing imported from .shp will look absolutely perfect and even will align well with other drawings. However, the datum used might be different than the default datum of WGS84 used in most modern GIS settings, such as GPS devices and Manifold. For example, many drawings in .shp format published by USGS will use the NAD27 datum. If we forget to change the datum after import we could introduce a small but possibly significant error into the data.
The message from the above is that when we verify the projection used by a new component we should not just launch the Assign Projection dialog and then blindly, credulously click the OK button without taking a moment to carefully review the dialog.
Manually Specifying Projection Information
Assuming we can lay our hands on the necessary projection information, we can import projected drawings or images from legacy formats. We first import using default settings for that format and then change the projection parameters from their default settings into the correct projection parameters to be used with that data. This is done on a one-time basis using the Edit - Assign Projection dialog.
To do this, click on the new drawing to highlight it in the project pane, and then open the Edit - Assign Projection dialog (or, open the drawing and then click on the verification info bar to open the dialog). This will display the (wrong) parameters used during default import. Enter the correct parameters intended to be used with that data. We only need do this once and then forever more Manifold will keep track of all projection parameters for us automatically.
Once a drawing or image is imported, the projection parameters in use for that component may be seen at any time by opening the Edit - Assign Projection dialog for that component. We can also reach this dialog by viewing the component's coordinate properties using View - Properties.
Changing the projection assigned to a drawing simply changes how Manifold interprets the internal coordinate numbers that comprise the actual data set. The only time we would do this manually is if we import a projected map from a legacy format and need to tell Manifold (manually) what parameters are supposed to be used. When importing a projected map from a legacy format, the internal coordinate numbers are already correct but there is nothing in the format that says how these numbers should be interpreted. We can tell Manifold how those numbers should be interpreted by entering them into the Assign Projection dialog on a one-time basis.
Importing Images
Regarding projections, most images fall into one of several classes:
· Simple overhead photographs created without any projection intended.
· Unprojected images intended for geographic usage.
· Projected images saved in smart geographic image formats.
· Projected images saved in legacy formats.
· Linked images served by an image server or linked from a data source.
Most images one encounters are simple photographic images in .jpg or other photo image formats. They usually show overhead aerial photographs of small geographic areas and are frequently oriented "north up." They are not intended as "projected" data sets, although most such images can often be handled as if they are projected in the Orthographic projection.
The Orthographic projection shows geographic data as a "view from space." When a map of the world or a hemisphere is shown in this projection, one sees the Earth as it would be seen from space. Zooming far into the center of such a projection provides a simulated ride towards the planet from space to a location directly overhead a particular location.
For all practical purposes, an aerial photograph shot from directly overhead a particular spot that covers only a few kilometers or miles may be treated as an image in Orthographic projection. By default, unless something about the format used tells Manifold to assign a specific projection, Manifold imports all images as if they are in Orthographic projection and automatically georeferences their lower left corner to the intersection of the Equator and the Prime Meridian (that is, zero latitude and zero longitude).
Note: Although the Orthographic projection is the classic "view from space" projection and thus widely used, one limitation of Orthographic is that the projection is defined only for the sphere datum. If other datums are to be used, an alternative "view from space" projection is the Stereographic projection.
To use North-up overhead images as part of a geographic map, we could use the Edit - Assign Projection dialog to "move" them to the right location. We could do this by moving the center of their projection (the lower left corner, in the case of images) as necessary to the right location and by adjusting the scale of the image. An easier way to accomplish the same thing is to use Manifold's georegistration tools to easily move, scale and warp the image as necessary from its default location. This is a lot easier than editing the coordinates properties by hand using the Assign Projection dialog.
Some images intended for geographic use cover very large areas of the Earth and are presented in "unprojected" Latitude / Longitude form. Using such images in Manifold is easy: we import them using default settings for images and then change the projection assigned in the Edit - Assign Projection dialog for that image to Latitude / Longitude. We will also have to set the scale factor in use. The scale factor can usually be determined from documentation or by computation (divide the number of degrees covered by the image by the number of pixels in the image to see how big each pixel is supposed to be). If desired, we can use the georegistration tools in Manifold or simply use trial and error to adjust the scale factor to be a good match to a known vector map.
Projected images that are saved in smart geographic formats are also easy to use. Manifold will import them with all necessary parameters with no user intervention required. All we need do is verify the projection used and we're done.
Projected images that are saved in legacy formats will waste more time than all the above cases put together. Such images usually fall into two subclasses:
· Images that are close to, but are not quite, overhead views and so must be adjusted.
· Sophisticated projected images saved in legacy formats that do not reliably capture projection information.
The first case is easy to deal with. We ignore the published projection information (if any) and assume the image is in Orthographic projection and then we force it into a shape that matches a known vector map by using Manifold tools to warp the image. When done in a sophisticated way, this is known as orthorectification. Many Landsat images, for example are very close to perfect overhead views. Even though such images are technically in Space Oblique Mercator projection, it is often much easier to treat them as Orthographic projection and to warp them slightly using Manifold georegistration tools than it is to track down the exact projection parameters that must be used.
The second case is more difficult to deal with and is directly analogous to encountering a projected drawing saved in a legacy GIS format. We need to find the projection parameters used in the projected image. We can then enter them into the Edit - Assign Projection dialog for this image. The problem here is that few GIS packages can handle projected images so often webmasters or librarians or other people who are involved in distributing such images realize that that projected images should be accompanied by projection parameters. Most folks will just treat a Landsat image, for example, as a simple photograph. When we ask for a Landsat image of a given area they simply give us a .jpg and don't realize that without the accompanying projection information it is not very useful in a GIS.
We can open such images in Manifold, but if we don't have the projection information we will not be able to re-project the image or otherwise use it in a sophisticated way. In the case of Landsat images, if we can track down the original copy of the image in the Landsat database we have a good chance of finding the projection parameters that must be used to use this image as a projected image within Space Oblique Projection.
Changing Projections
Once a drawing or image is correctly imported into Manifold and the projection has been verified we can change the projection into whatever projection we like. To re-project a drawing into a new coordinate system we use the Edit - Change Projection dialog.
The dialog opens with the projection parameters currently in use. These are taken from the coordinates properties of the drawing. If the drawing was imported correctly (with manual updating of coordinate properties if need be using the Assign Projection dialog), these parameters will be the correct parameters required for the coordinates in the drawing to make sense.
When we use the Edit - Change Projection dialog to cast the drawing into a new projection, Manifold will re-compute the coordinates within the drawing to the equivalent numbers required by the new projection. Manifold will also change the coordinates properties to the new parameters now required to make sense of the new numbers. All of this happens automatically. Henceforth, when we open either the coordinates properties dialog or the Assign Projections dialog we will see new parameters to match the new coordinate numbers. Re-projecting a drawing in this way permanently changes the internal coordinates used to define the drawing.
Because drawings are often seen through a map window within maps, it is not always necessary to re-project them if we want to see the drawing in a different projection. We can leave the drawing in whatever projection it was in on import and view it using a map that has been assigned a different projection. The map will re-compute the view on the fly to present the drawing as it would appear in that different projection.
Specifying the projection used by a map does not change any data inside the drawings - it simply changes the way the data is seen by computing on the fly how the drawing should look in the projection requested of that map. Even if several different drawings using different native projections participate in a map Manifold can re-compute them all on the fly to display them correctly in the projection desired for that map. As long as the drawings are not so large that our computer slows down too much during the re-computation process, this works well.
If we are working with large drawings and they must appear within a map using a different projection the process of displaying them in the map may be too slow for our taste. In that case, we can either change the projection of the map to be the same as the drawing, or we can re-project the drawing into the projection used by the map. A map window can work faster if the drawings and images they contain use the same projection requested of the map. This is especially true if many large drawings and images are used in the map.
As explained in detail in the Images topic, images may also be re-projected Images that are used within maps will often be re-projected to match the projection requested of the map. This is because images tend to involve much more data than drawings (each pixel in an image is like an object in a drawing) and re-projecting them on the fly is slower than re-projecting drawings.
A Projections Strategy
For most interactive editing and map preparation work in middle latitudes a good strategy is to keep all drawings in Latitude / Longitude form and to work with them in maps using the Latitude / Longitude projection. This is the name given to the "unprojection" that uses simple latitude and longitude degrees instead of any other projection. Most GIS data is published in this form, so it's likely that this will be the native "projection" used by most drawings on import. Latitude / Longitude projection is also used to publish most images that cover the entire Earth, such as the globe.bmp sample image shown above.
When working within smaller regions or individual cities, working in Orthographic projection centered on the region of interest is often a good choice. This is the default projection used for image and non-geographic drawing import, so it is fast even if images are involved. It is also the closest match to an overhead view as would be seen from an airplane flying over the region.
Of course, if you have a fast system or are working with smaller maps you can simply keep drawings and images in whatever projection they were in when imported and use whatever projection you like in maps.
Very Important: To re-project a component, use the Edit - Change Projection command. Do NOT use the Edit - Assign Projection dialog. This latter command is used to manually correct the projection of a component imported from a format that is unable to provide projection information.
Notes
· As discussed in the Coordinates topics, CAD drawings coming into a map from non-geographic contexts will have to be georeferenced the way images must be georeferenced. Like images, CAD drawings also use the Orthographic projection by default and are georeferenced in the same way.
· Recent extensions to ESRI .shp format will save partial projection information within a .prj auxiliary file; however, most .shp data predates the availability of this latest hack so it does not help much.
· Although the georeferencing tools are a lot easier to use than editing an image's coordinates properties by hand, every expert user of Manifold should experiment with hand-editing coordinates properties to better learn how coordinates work.
· We must apologize for such a large, dense topic so early in the Introduction. However, it is made necessary by the prevalence of legacy GIS formats. Working in smart GIS formats is easy and requires much less expertise in projections.
See Also