I imported that DEM file in R using the following commands:

# DEM file

# Set working directory
  setwd("C:\\users\\jbaldwin\\Downloads")

# Load library
  library(rgdal)

# Input data
  g = readGDAL("dem_3009147ne.dem")

# Get coordinates
  xy = coordinates(g)

# Show the structure of g
  str(g)

So getting the coordinates (in UTM's) can be done by running R from Mathematica. Alternatively by comparing what R gets and the elements that Mathematica imports, one can determine the UTM's of any element of the data matrix with

(* Get necessary elements of DEM file *)
elev = Import["C:\\Users\\jbaldwin\\Downloads\\dem_3009147ne.dem", 
   "Data"];
dim = Import["C:\\Users\\jbaldwin\\Downloads\\dem_3009147ne.dem", 
  "Dimensions"]
resolution = 
 Import["C:\\Users\\jbaldwin\\Downloads\\dem_3009147ne.dem", 
  "SpatialResolution"]
boundingBox = 
 Import["C:\\Users\\jbaldwin\\Downloads\\dem_3009147ne.dem", 
  "SpatialRange"]

followed by

(* Get elevation (in meters) and coordinates (in UTMs) for a \
particular element *)
i = 1291
j = 350
elev[[i, j]]
utmx = boundingBox[[2, 1]] + i resolution[[1]]
utmy = boundingBox[[1, 1]] + j resolution[[2]]

or a something like a Table that gets all of the UTM's for each row and each column. You'll be getting UTM's rather than latitudes and longitudes. Also, the above R code ends up with the elevation in feet. Either there's some flag in the file that says it's in feet and R keeps it in feet or Mathematica always converts to meters for this particular file format irrespective of that flag or certainly some other rule. I don't know.

Interpolating the data set contained in the DEM file is quite straightforward (see the earlier post above) as is getting the other data elements contained in the file. The key questions that are being raised here are first, how to get the actual latitude and longitude range from the file. The data elements do not appear to contain this. The second issue is, since the data represents a grid for a particular map projection, how to invert that to actually get the latitude the height for specific latitude and longitude points once those ranges are known.

I am trying to see if maybe I can kludge a solution. The following demonstrates the ability to outline a parish (county) as a polygon.

GeoGraphics[{GeoStyling["StreetMap"], 
  Polygon[Entity["AdministrativeDivision", 
         {"EastBatonRouge", "Louisiana", "UnitedStates"}]]}, 
 GeoBackground -> None]

If one double clicks and double clicks again in the center of the image all the nodes of the polygon are highlighted. What I haven't been able to do is to tease out the?coordinates of the nodes. With a matrix of the coordinates I may be able to recursively test inside the polygon for the characteristics I am looking for. Then I can repeat for other parishes and counties.

I've played with Entities and GeoEntities without any luck, so far.

My earlier hope was to simply use a matrix of the height values and coordinates (bounded by the polygon) for testing. If one knows a coordinate the height is available. Taking many tthousands ?of points is beyond manual input methods.

JAC

Could you attach your example file so that we have a chance to play with it and see what we can suggest?