An orthographic projection is an unusual type of projection where the map is transformed to look like how the earth would appear to an observer viewing it from a distance. This is more akin to a perspective drawing than a map. There is an issue with orthographic projections in MAPublisher where areas that are on the “back” of the globe overlap areas on the “front”. We have come up with a workaround for this issue where you will crop to the valid extents of the projection before transforming the data. This way the overlapping polygons will be cropped out before the data is transformed.
Note that you may be able to use this workaround with other transformations where you are seeing similar issues. It occasionally happens with custom projections where the central meridian has been adjusted. Any time you have either overlapping polygons or vertices stretching from one side of the map to the other, this workaround may fix the issue.
To see this error in MAPublisher and to follow these instructions, download this Illustrator document. Open orthographic_transformation.ai and examine the map. There are a few obvious problems. Australia, Southeast Asia, and Oceania are all overlapping Africa. Russia is overlapping Europe, and islands in the South Pacific seem to be in the Atlantic Ocean.
Overlapping polygons at various locations
To rectify this, we will create a graticule, convert it to a polygon covering only the front part of the globe, then transform that polygon to WGS 84 and use it to crop the un-transformed, WGS 84 layer. Then we will transform that layer to an orthographic projection.
- Create a graticule for the Orthographic map view. Under Grid Border, un-check the option Draw Border. Leave the other settings as default
- Expand the graticule to break it up into its constituent objects. Select the graticule and go to Object > Expand. Ensure all the options are checked and click OK.
- Open the Layers panel, expand the group layers Group and Clip Group under Legend, and scroll down to the bottom. Delete the group layers InterceptClipGroup and KnockoutClipGroup
- Open the MAP Views panel and drag the layer Legend from the Orthographic MAP View to Non-MAP Layers. When a warning message appears, click Remove MAP data
- Drag the Legend layer to the Orthographic MAP View to convert the graticule to a polygon layer. When the message box appears to define the feature type, select Area and click OK
- Select all objects on the Legend layer and open the Join Areas tool. for Destination select Join areas on target layer, for Join Type select Join all selected areas on target layer, for Join method select Create compound areas, and check the box Dissolve borders between adjacent sub-areas. All of the objects on the Legend layer are now merged into a single circular polygon covering the extents of the front of the globe. Change the stroke color of this layer to make it visible
- Drag the Legend layer to the WGS 84 MAP View transform it to WGS 84 and align it with the data in this MAP View. Toggle the visibility off for continent_area and on for continent_area 1 to see how the polygon lines up with the WGS 84 data
- Select the polygon object on the Legend layer and run Crop to Shape. Crop All layers in WGS 84 and click OK
World map in WGS 84 cropped to valid extents for the orthographic projection
- Drag the layer continent_area 1 to the MAP View Orthographic to transform the data to an orthographic projection
With the areas outside of the valid extent cropped out, the map is displayed properly without overlapping polygons.