Using apps.nationalmap.gov/3depdem to display LiDAR data
                by Roy Schweiker


Web address is:	https://apps.nationalmap.gov/3depdem/   This displays best on large screens, some features seem to disappear on small screens.

To display contours, select the 7th icon from the top, on the left margin of the 3depdem viewer. A box at upper left should appear, allowing you to select the contour interval, with a minimum of 1 meter. Note that the contour lines are not at even meter elevations, they may be at 999.72 and 1000.72 for instance.

You can attempt to keep track of your location by selecting a basemap with the Basemap Gallery icon (four squares, under the "+" and "-" icons), then using the Transparency Slider (9th icon, balance scales) to set to 50% or more to allow basemaps to display underneath. I don't find that this works well, and prefer to keep a regular topo map open in another window to aid navigation.

Locations can be entered as names or coordinates, but the preferred format is like Y:44.500000 X:-70.500000 Other formats may display the correct place, the wrong place, or give you a menu of choices. Use the "+" box to zoom in. Note that you will probably not come up within the highest contour, and will have to look around.

Click on the "i" icon in the left column, then click on a point on the map. A box will come up with info on that point.

Important: Look under Title to see what dataset is being used. If it is LiDAR the title will include something like "one meter", while if it says "1/3 arc-second" this is not LiDAR but just scaled from USGS maps and not very accurate. Even if LiDAR has been done for an area, it may not be loaded into this viewer yet.

If you have LiDAR data, each pixel represents a 1-meter square. To find the guaranteed highest elevation you would need to click on every pixel within the highest contour (and maybe a few outside), a near-impossible task. If you care that much you should probably download the dataset and search for the highest point in the area. Otherwise a few clicks will give you the approximate highest elevation. Finding a col elevation is similar but probably a little less accurate. There may be multiple highest-contour points and maybe multiple cols.

Example: Markham Mtn in VT with 3 summit areas on USGS map http://mapper.acme.com/?ll=43.27409,-72.75871&z=15&t=U&marker0=43.27409%2C-72.75871%2C43.2741%25C2%25B0N%2072.7587%25C2%25B0W

I find that the S peak has an approximate elevation of 2497 feet, and the middle peak with the register is 2503. Someone earlier claimed that the N peak is higher yet and LiDAR data seems to bear this out. The trail passes between 2 bumps, which LiDAR calls 2509 (west) and 2510 (east). You would need to download to review the dataset to see which has the highest point, and then consider possible LiDAR errors. Fortunately these bumps are close together and it's easy to visit both bumps. LiDAR believers may rule out the middle (register) peak and the S peak is even farther out of contention.