Mollweide Interrupted



A pseudocylindrical, equal area projection. Also known as Homolographic projection, Homalographic projection, Babinet projection, and Elliptical projection. The Molleweide Interrupted projection is a variation of the Mollweide projection.


The Clip Coordinates checkbox must be checked when projecting maps into Mollweide Interrupted.




True along latitudes 40°44’ North and South.


Scale is constant along any given latitude and the same for the latitude of opposite sign.




Free of distortion only at latitudes 40°44’ North and South on the central meridian. Distortion is severe near outer meridians at high latitudes.




The Mollweide Interrupted is used to display the oceans of the world. For example, for voyagers it's a nearly perfect projection to show the path of a circumnavigation via the Panama Canal and the Red Sea.




Presented by Carl B. Mollweide (1774 - 1825) of Germany in 1805.




Clip Coordinates must be checked.


Tech Tip


Interrupted projections are not continuous coordinate systems. They employ multiple conversion domains with blank space between the different lobes of the conversion domains. It is absolutely essential to check the Clip Coordinates box in the Projection dialog whenever using such projections. This box causes Manifold to clip each object so that it exists only within the allowed conversion domain lobes and does not extend or cross through disallowed blank space. This is a highly computationally intensive process so the Clip Coordinates box is not checked by default, so that significant overhead is not imposed if it is unnecessary.


Dealing with the separate conversion domain lobes of an interrupted projection requires a manual approach to creating graticules, since the graticule lines normally extend through the blank space between lobes.


Use the following procedure:


1. Create a latitude / longitude drawing.

2. Use the View - Graticule tool to create the desired graticule, using the option to Create the graticule as line objects.

3. Project the drawing into the desired interrupted projection, making sure to check Clip Coordinates.

4. Edit the graticule lines by selecting undesired lines and deleting them, or by adding lines.


For many uses the fastest method is to add lines. Suppose we begin with a latitude / longitude drawing in which a graticule was created with lines every 10 degrees from -180 to 180 longitude and from -70 to 70 latitude.




After projection into Mollweide Interrupted with Clip Coordinates checked, the lines that appear on the edges of the conversion domain lobes will have been deleted.




images\btn_snapto_lines.gif images\btn_shp_line.gif We can add lines by clicking Snap To Lines and then using the Insert Line tool to add lines between the "dangling" parallels. This goes very rapidly with less than a minute required to complete the graticule.




The result will be a graticule grid with lines restored that were deleted by Clip Coordinates. This procedure was used to create the graticule seen in the interrupted projection illustration above.