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I was going to go with "Divide the land area of a state by the area of its convex hull. Which is the lowest percentage?"

by howeman 13y ago

mjw 13y ago
Yeah that was what I was expecting they'd do also. Maybe you'd want to compute the convex hull on a per-connected-component basis, though, to avoid allowing small islands to have undue influence. Either that, or say that any area of sea within its convex hull gets treated as part of the state's area. This approach differs in that it special-cases sea over other kinds of land.
- lessnonymous 13y ago
  Would you not get the mean height above sea level of all points along the border and divide that by the mean height above sea level of the entire state (which I guess is really an average of some sampling of points)?
  - mjw 13y ago
    For that suggestion I was just considering it a two-dimensional problem with a "sea or not sea" distinction, not using heights above sea level.
tzs 13y ago
My first thought was to go with the length of the state's boundary that is on the convex hull divided by the length of the whole boundary. So...we've got at least 3 ways so far to assign a measure of convexity/concavity to a state, and they give quite different results. For instance, your method and the method in the comment you responded to both would assign a low concavity to a state that is almost a square, except that the boundary has a high frequency, low amplitude sawtooth pattern imposed on it. Mine would score that has a very high concavity.
- thejteam 13y ago
  I was interested in this and did a very quick google search. Measuring concavity/convexity of a generic 2D surface object doesn't seem to be a very common operation. The one reference I found seems to do what you describe: http://www.ncbi.nlm.nih.gov/pubmed/7945702 http://www.ncbi.nlm.nih.gov/pubmed/7945702 Although I don't have PubMed access so I can't be sure. This is an interesting problem. I chose the "random points" method because it seemed easier to me to get an approximate answer for a generic shape rather than try to figure out areas. Lengths didn't occur to me though and seems interesting, although the shape you describe doesn't "feel" concave to me.

I was going to go with "Divide the land area of a state by the area of its convex hull. Which is the lowest percentage?"

by howeman 13y ago

I was going to go with "Divide the land area of a state by the area of its convex hull. Which is the lowest percentage?"

mjw 13y ago

Yeah that was what I was expecting they'd do also. Maybe you'd want to compute the convex hull on a per-connected-component basis, though, to avoid allowing small islands to have undue influence. Either that, or say that any area of sea within its convex hull gets treated as part of the state's area. This approach differs in that it special-cases sea over other kinds of land.

lessnonymous 13y ago
Would you not get the mean height above sea level of all points along the border and divide that by the mean height above sea level of the entire state (which I guess is really an average of some sampling of points)?
- mjw 13y ago
  For that suggestion I was just considering it a two-dimensional problem with a "sea or not sea" distinction, not using heights above sea level.

tzs 13y ago

My first thought was to go with the length of the state's boundary that is on the convex hull divided by the length of the whole boundary. So...we've got at least 3 ways so far to assign a measure of convexity/concavity to a state, and they give quite different results. For instance, your method and the method in the comment you responded to both would assign a low concavity to a state that is almost a square, except that the boundary has a high frequency, low amplitude sawtooth pattern imposed on it. Mine would score that has a very high concavity.

thejteam 13y ago
I was interested in this and did a very quick google search. Measuring concavity/convexity of a generic 2D surface object doesn't seem to be a very common operation. The one reference I found seems to do what you describe: http://www.ncbi.nlm.nih.gov/pubmed/7945702 http://www.ncbi.nlm.nih.gov/pubmed/7945702 Although I don't have PubMed access so I can't be sure. This is an interesting problem. I chose the "random points" method because it seemed easier to me to get an approximate answer for a generic shape rather than try to figure out areas. Lengths didn't occur to me though and seems interesting, although the shape you describe doesn't "feel" concave to me.