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.

Do any of the WRI developers on the forum have any advice for Mr. Cavell? The main issue is, given a DEM file, read it in and generate a function height[latitude,longitude] based on an interpolation of the supplied grid data in the DEM file. The two things that need to be extracted and analyzed for this are

a) the latitude and longitude range of the DEM array data b) any transformations that need to be done on the data based on the map projection indicated in the DEM file

It would be a good addition to DEM import if such a function could be generated as one of the import Elements for DEM

I wish to compliment Dr. Reiss for his help. I contacted him off-line because I had a follow-up question that involved a brand name other than Wolfram and didn't want to be impolite to our host. He has suggested a few ideas that seem to be getting me closer to a possible solution.

file = "...local folders.../dem_3009147ne.dem"
Import[file, "ReliefImage"]
interp = ListInterpolation[Transpose@data]
interp["Domain"]
{{1., 1225.}, {1., 1405.}}
DensityPlot[interp[x, y], {x, 1, 1225}, {y, 1, 1405}, PlotPoints -> 100]

He then suggested I revisit the GeoElevationData function, which I did. Providing a lat, log it returns a value for height, but I am wary of it because I haven't found a quotable statement of the source of that elevation result. One reason I was investigating the DEM was I know its pedigree and can stand behind the results.

A point on the peninsula looking crook in the river is 30°19'48.13"N lat, 91° 9'38.35"W lon (30.33003611, -91.16065278)

GeoElevationData[{30.33003611, -91.16065278}]

returns:

Quantity[55.7743, "Feet"]

which may be an OK value but I can't be sure. The grid on which it is based is pretty coarse compared to the DEM and when I checked specific sites in urban areas the discrepancies cause some concern.

JAC

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