Edit - Assign Projection

The Edit - Assign Projection dialog shows the coordinate system (projection) settings Manifold uses to interpret the coordinate numbers currently within a drawing or used with the pixels in an image. Changes in this dialog change merely the interpretation of the numbers and not the actual coordinate numbers or arrangement of pixels.

 

This dialog is also used to assign a projection to be used by a map. In this case, it tells the map what projection to use to display its contents. If the component layers in the map use different native projections, the map will re-project the components on the fly to show them as they would appear in the projection assigned to the map.

 

Note: This dialog is normally called just once when importing drawings or images from insensate formats. It is not the dialog to use to "re-project" a drawing or to specify a projection desired for a map. To re-project a drawing, image, or other component, use the Edit - Change Projection dialog.

 

The only time this dialog is used with components other than maps is to either verify the projection is correct or to specify the correct parameters after importing a projected component from a legacy format that does not contain projection parameter information. The component is imported as though it was unprojected (no parameters involved) and then this dialog is used to manually set the correct coordinate system parameters. Do not make changes in this dialog unless you are sure that the component was imported from a legacy format and now needs to have the correct coordinate system / projection parameters entered manually.

 

Although this dialog is visually similar to the Edit - Change Projection dialog (because both deal with similar data), the two are critically different in function. The Edit - Assign Projection dialog tells the system how to interpret existing data without modifying the data. The Edit - Change Projection dialog changes the data to fit a required interpretation.

 

The Edit - Assign Projection dialog is used to make changes on a "one time" basis only when an import from a legacy format requires manual entry of the correct projection parameters. Changing parameters in the Edit - Assign Projection dialog changes only the interpretation of the existing data and not the coordinate data.

 

The Edit - Change Projection dialog, in contrast, changes both the actual coordinate numbers as well as their interpretation. It is used every time one desires to recast a drawing or image into a new projection.

 

images\dlg_assign_projection.gif

 

The dialog includes these controls:

 

images\xbtn_load_file.gif

Load from File - Load projection information from a file using ERDAS AUX, Golden Software GSR, ESRI PRJ, or XML files.

images\xbtn_save_file.gif

Save to File - Save projection information to a file using XML, Golden Software GSR or ESRI PRJ files.

images\xbtn_recent_settings.gif

Recent settings - Choose a projection setting to reuse from a list of recently-used projections employed in this Manifold session.

images\xbtn_load_from_component.gif

Load from Component - Load projection settings from another component in this project.

(Projection)

A tree diagram providing numerous projections within various types of projections.

(Datum)

A list of standard datums that incorporate various Earth ellipsoids and standard offsets.

(Parameters)

One or more optional parameter settings, if used by this particular projection. Double-click into the values boxes to change them. Press Enter after changing a value.

(units box)

Choose the unit of measure. All "unprojected" maps are in Degrees, Arc Minutes or Arc Seconds. Most projected maps are in meters.

Local offset

Shifted version of the external coordinate system peculiar to this component. Specifies shift in X (longitude) and Y (latitude) direction in the given units. With images or surfaces, the Local offset stores the offset to the left-bottom corner of the left-bottom pixel of the image or surface.

Local scale

Scaled version of the external coordinate system peculiar to this component. Specifies scale factor to be applied to external coordinate system.

Scale correction

Controls scale of the external coordinate system. For example, 0.9996 for UTM.

False easting / northing

Shift of the external coordinate system. For example, 500,000 meters for UTM. Some projections (such as UTM) add values to the "true" X and Y coordinates so that all coordinates stored are positive numbers. This avoids the use of the minus sign in primitive formats. The value added to X is the Easting and that to the Y is the Northing.

Adjust for units

Automatically adjust easting/northing values when units of measure are changed. Not checked by default.

Clip coordinates

Not used in the Edit - Assign Projection dialog.

Autosuggest local values

Not used in the Edit - Assign Projection dialog.

Preserve local values

Normally, choosing a projection from the projections pane will reset the values of Local offset, Local scale and units to the preset values for that projection. Checking this box will preserve any existing values in these local fields even when a new projection is selected. It is checked by default in the Edit - Assign Projection dialog because this dialog is often used to manually specify projection information when importing from geographically unaware legacy formats. In that case, any custom specifications for local values should probably be preserved when changing the projection.

Suggest

Suggest values for projection parameters based on the view in the component window. Used with projections with user-specifiable parameters.

 

The most frequent use of this dialog is to manually provide projection information when importing from formats too geographically unaware to provide such information (such as .shp or .dxf). After importing projected drawings from such files, the user must provide the missing projection information using the Edit - Assign Projection dialog.

 

To import a projected drawing or image from a legacy format:

 

1. Import the drawing or image using File - Import. Use default settings.

2. Open the drawing or image.

3. Use the Edit - Assign Projection dialog to specify the projection information that should be used.

 

The Edit - Assign Projection dialog allows us to manually provide the correct projection information into the coordinates properties. Manifold will then be able to make sense of the data just imported.

 

Do not confuse the assignment of a chosen projection view in a map window (via Edit - Assign Projection ) with the use of Edit - Assign Projection in a drawing window to specify projection information missing from legacy formats. If a projected drawing is imported from, say, .shp format and you fail to tell Manifold the correct projection to use, that drawing has not yet been correctly imported. Subsequent use of the drawing in maps may cause bizarre effects or lengthy delays as the system attempts to compute a new projection for the map based on fundamentally inaccurate coordinates.

 

Note: Maps can work faster if the drawings and images they contain as layers use the same projection that is requested of the map. If we always use a particular projection within a map, it will be worth it to re-project the constituent drawings and images of that map into that particular projection as well. To permanently re-project a drawing or image, use the Edit - Change Projection dialog.

 

Adjust for units Option

 

When manually changing units of measure the false easting and northing values will not be changed correspondingly. Checking the Adjust for units box will force a recomputation of the easting and northing values into the corresponding values for the new units of measure. For example, to change from Feet to Meters as units of measure and to change easting / northing accordingly:

 

·      Verify the Adjust for units option is off,

·      Set the units box to Foot,

·      Turn on the Adjust for units option,

·      Set the units box to Meter.

 

Projection Toolbar Buttons

 

We may want to re-use the same projection information with different components. For example, suppose we have many shapefiles covering the same region of interest that contain projected information requiring the use of the Edit - Assign Projection dialog after import. Since all of the files will require the same projection information, after importing one file and manually specifying the projection settings in the Edit - Assign Projection dialog we can save the projection information using the Save to File toolbar button. With subsequent files we can then use the Load from File button to load the settings and thus avoid manual re-entry of repetitive information.

 

Another time saver is to load projection information from an existing component using the Load from Component button. For example, we may want to re-project all files into a certain standard projection used by a given component and would like to be sure that all settings are identical. In that case, we open each component to be re-projected, launch the Edit - Change Projection dialog and use Load from Component to load the desired settings from the component we wish to use as the standard.

 

Inadvertent Usage

 

With components other than maps, the Assign Projection dialog should be used only once: when the component is first used to verify the projection is correct or to assign the correct projection if it was not correct. Using this projection in other circumstances is almost certainly a mistake.

 

In particular, it does not make sense to first verify the projection using the Assign Projection dialog, then change the projection using the Change Projection dialog and then finally to use the Assign Projection dialog. Once the projection is verified to be correct and then the projection is changed, it simply does not make sense to, in effect, state that the result is now wrong by attempting to use Assign Projection once more.

 

If we do attempt to use Assign Projection after doing a Change Projection, then Manifold will raise a warning message. If we elect to proceed, Manifold will allow us to continue as there are rare circumstances when expert usage may indicate a very minor change to a projection using Assign Projection after a Change Projection has been done. If we are in the habit of doing such exotic and expert things we can turn off the warning message by un-checking the Confirm re-assigning projection after projecting data option in the Tools - Options dialog.

 

Additional Reading

 

Experts may jump directly to the Projections Quick Reference topic. See the Edit - Change Projection topic for a discussion of the Local scale, Local offset, Scale correction and False easting/northing options. These are expert level parameters intended for professional use.

 

If you are new to coordinate systems in GIS or would like a refresher on coordinate systems and projections, consult the Coordinates Tutorial , the Projections Tutorial , and the Coordinates in Projected Maps topics followed by a quick review of the Projections Quick Reference topic for a summary.

 

See an example of usage in the Manually Georegister an Image example topic, in the Import a Projected Shapefile topic and in the Download and Mosaic Terraserver Images topic.

 

Differences between Edit - Assign Projection and Edit - Change Projection

 

·      Edit - Assign Projection changes only the view of the data in maps and the interpretation of the data in other windows. It does not change the actual coordinates in the data. With map windows, it changes the projection used in the view without actually changing the data in the components. Map windows have dynamic projections that can show their contents in whatever projection is desired regardless of the native projections of what is inside them. When used with a drawing, image, labels or surface window it changes the interpretation of the data. This is a rarely used function that is utilized only to manually adjust projection properties when importing from formats that do not correctly store projection information.

·      Edit - Change Projection appears with individual component windows such as drawing, image, labels or surface windows. This command changes the actual data in the component into a new coordinate system. There is no ability in a drawing window to see the drawing in a projection other than its native projection. We use Edit - Change Projection to re-project a component.

 

Notes

 

When opened with a read-only component (such as a drawing or other component stored on an Enterprise server when using Enterprise Edition ), projection dialogs will report projection information but will not allow changes.

 

The Assign Projection and Change Projection dialogs also appear in the project pane context menu (right click on a component and the pop-up context menu appears). These dialogs will not be enabled in project pane context menus for components that take their projections automatically from parent components, for example, labels components bound to drawings or profiles.

 

See Also

 

Edit - Change Projection