topic Re: antipodal line in convex hull in ArcGIS Pro Questions

antipodal line in convex hull

EstebanRodofili — Sat, 25 Jun 2022 01:28:46 GMT

Hi! I am using the convex hull option in the minimum bounding geometry tool in ArcGIS Pro, to obtain the antipodal line and its orientation. While I get the orientation, the length of the line is zero and the coordinates of the antipodal points are the same (see screen capture). Is it that my features are too small? How could this be solved? I would like to have the points at the ends of the line to recreate that line as another feature. Thanks

Re: antipodal line in convex hull

DanLee — Sat, 25 Jun 2022 03:01:45 GMT

I am guessing your data is in decimal degrees. So for a small island, the length value would be too small and perhaps got rounded up to 0 degree. You can open the Field View of the table, find the field and increase the decimal places to 15 (maximum). Save the change. Go back to your table to see if a very small value shows up.

Re: antipodal line in convex hull

EstebanRodofili — Sat, 25 Jun 2022 23:28:03 GMT

Thanks! That worked! However, when I went to distance and direction to create the antipodal line from the coordinates, a line much longer than it should be appeared. And the type of coordinates selected in the distance and direction tool was decimal degrees. Any idea what could be happening?

Re: antipodal line in convex hull

DanLee — Sun, 26 Jun 2022 01:26:22 GMT

Could you show details on how you "went to distance and direction to create the line" with screenshot or interactive steps?

To do spatial analysis on this seemingly an island, the best practice is to project your data so they are in the projected coordinates and distances are in meters or the relevant units. Using distance in very small decimal degrees can result in unexpected result.

Re: antipodal line in convex hull

EstebanRodofili — Tue, 28 Jun 2022 17:09:25 GMT

Just to save us time, essentially what I am trying to do is to visualize the antipodal line. These features correspond to something smaller than islands. They are manatees in drone images. I am just trying to see the antipodal line to discard it as a tool. What I am really after is a line that goes from head to tail directly, in order to get its orientation and hence the orientation of the animal. However, the antipodal line is the longest line connecting points in the perimeter of a polygon you can find, right? That means most of the time, if a manatee has a tear drop shape, it will not go from the head to the tail in a straight way, more to the end of the animal but to one side, in a diagonal direction, as it is the longest possible line inside the polygon?