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# Two related questions about GeoGraphics

Posted 1 year ago
 (1) Is it possible to get the list of points in Polygon[entity], where entity is some geographic entity with an associated polygon defining its boundaries? (2) Is there a way to define a geographic region in a way that generalizes GeoDisk? Specifically, a region that lies within, e.g., 50 miles of the city limits of a given city (mathematically, the border of the region is the envelope of the circles centered on the points of the city limits). I could set up an implicit definition using GeoDistance (I think), but would I be able to plot that using GeoGraphics? Thanks for considering. Ron
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Posted 1 year ago
 (1) is exactly what I need. Thank you! I should have come up with that myself.(2) is more complicated, I think. You have a single disk centered on the position associated with NYC, (presumably a centroid, I imagine). What I want would be the union of all the disks centered on the points on the city limits, i.e., the points you collected in "points" in (1), and all the points along the segments joining those points. It would be easy to write a mathematical description of this set, but I'm not sure how to get it in a form where GeoGraphics can use it.
Posted 1 year ago
 Question (2) may be similar to the concept of "dilation of a region". It has a geographic example in the documentation of RegionDistance.
Posted 1 year ago
 Thanks, that does look promising. I haven't used that function before, but this definitely gives me something to go on. As long as I can specify the metric (it is non-Euclidean in this case), I can probably work in the latitude-longitude parameter plane with the appropriate metric to find the boundary points of the region.
Posted 1 year ago
 I remember I have done something like that to mark contour lines defined by the distance from a coast. First I map the land onto a sphere in 3D, where I can use EuclideanDistance. From that I define a function of latitude-longitude and make a ContourPlot. Finally I map the contours into GeoGraphics inserting GeoPosition between Line[ and the list of points.
Posted 1 year ago
 For (1) polygon = Entity["City", {"NewYork", "NewYork", "UnitedStates"}]["Polygon"] points = List @@ First@polygon For (2), do you mean something like this? nycPosition = Entity["City", {"NewYork", "NewYork", "UnitedStates"}]["Position"] GeoGraphics[GeoDisk[nycPosition, Quantity[50, "Miles"]]]