Adding more area to a feature lowers the area apparently?

Bug Reports
1

im fairly certain this may be a bug of some kind or maybe im doing something wrong??? im still somewhat new to gplates so forgive any beginner mistakes

in some cases, adding terrain will decrease the area of the feature, and othertimes it will act normally and state that ive added more terrain, i cant do much since im a new user and it’ll only add one embed at a time which cant do much in terms of illustrating my point

does anybody know what’s going on? am i doing something wrong or is this actually a bug?

edit: I just realised I can just link my twitter post about the issue here and that should be fine https://twitter.com/PPaleoartist/status/1726996318705635575?s=20

3

Can you send me the before and after polygons used in your twitter post?

4

sorry about my beginnerness but how do i do that?

5

No problem - I’ve sent you a PM with my email address - you can attach them to an email to me.

6

Thanks for sending me the polygons.

It’s actually due to the orientation of the sub-parts of your polygon (you have a single polygon consisting of 5 sub-parts joined together). In the following image sub-parts 1 and 4 are counter-clockwise whereas 2, 3 and 5 are clockwise. This means increasing the size of 3 will actually decrease the total polygon area (while increasing the size of 4 will increase the total area). I verified this by digitizing 5 separate polygons and adding their areas as 1-2-3+4-5 and it came out to the area you had (in your twitter post).

image
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Also note that it doesn’t matter if you digitisized your single large polygon in the opposite direction (ie, 1 and 4 clockwise and 2,3,5 counter-clockwise). Increasing the size of 3 would still decrease the total area.

So in your case it might be better to either digitize all the sub-parts of your polygon with the same orientation, or digitize 5 separate polygons.

This is all due to the way polygon area is calculated in GPlates. It’s done by forming a spherical triangle for each polygon edge (ie, two vertices) and the polygon centroid. And then calculating the sum of the signed areas of these triangles. And any interior rings have the opposite sign to the exterior ring (to ensure they subtract area from the exterior ring) - but your polygon does not have any interior rings.

1 Like
7

Ah I get it now thank you so much for your help!