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In my map showing Antarctica (projection = polar stereographic) how do I get rid of the seam?

August 01 2007 | 0 comments
Categories: ArcGIS Methods, Map Data

When using polar projections in ArcMap, I often get artifact lines as in the attached graphic.
(In this case, the projection is WGS 1984 Antarctic Polar Stereographic.) They look like maybe they correspond to the prime meridian or some meridian. Refreshing the map, turning the layer on and off, etc. doesn't seem to help. I don't know if this quite counts as a cartographic question, but is there a way to deal with it? Thanks.

Attachment: Antarctica in polar projection

Mapping Center Answer:

The artifact lines or seams are actually in your data. The typical case is that data is stored in a geographic coordinate system, e.g., WGS84, and to do that, a seam is introduced into data where it intersects the 180 degree line—it’s easiest to see this in Antarctica. Thus, when you change the projection of your data frame, particularly to something suitable for showing Antarctica, the seam in the data is projected too—becoming quite obvious.

To eliminate the seam, you’ll need to project the data and then edit it. For the projecting, I usually create a new feature dataset in my geodatabase that is set to the correct project—as I’m typically doing several datasets, and that saves me time and reduces the potential for error. For the editing, I start editing and select the feature, change the edit task to Modify, and using the select tool I hover over the vertex that anchors the seam and then right-click. The resulting menu should have the choice to remove the vertex enabled (sometimes the editor is fussy about where you right-click and the choice won’t be enabled). Save the changes and now you’ve got a “seamless” version of the data to work with.

In rare cases you can actually introduce holes (interior parts) into you geometry when you change the projection. To remove the part, start editing, select the shape, and open the Sketch properties window (rightmost button on the Editor toolbar). Select each part until you see (in the data frame) the offending geometry highlight. The in the Sketch properties window, right-click on that row in the part list and delete the part.

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