Tag Archives: topography

USGS Mapping

I haven’t been in the US for more than half a year and I’m not going for another few months. But I’m not wasting my time. Instead I’m preparing another awesome trip for the autumn. At this stage I spend a lot of my free time studying maps thinking about future road-trips.

So, maps: let me share some thoughts about the subject.

Obviously everyone is aware of Google maps. This is great tool for route planning. It calculates distances, driving times and lets you look at street level (useful when you are trying to work out how to exit unfamiliar airport parking or drive to a small motel hidden down a side street). But as much as I like Google, its mapping has some obvious limitations.

Probably the most important is the lack of contour lines or other topographic information. You really can’t work out how steep the road is or sometimes even if the terrain is flat or hilly. For that you need some proper topographic maps.

This is where the USGS comes in, which stands for the United States Geological Survey. Created by an act of congress in 1879 it is the agency of the United States government which studies landscape, geology, natural resources and natural hazards. It is like joining the Ordinance Survey and British Geological Survey into one agency and then throwing in a few more bits and pieces.

USGS produces many map series, both topographical and geological. The largest, most detailed and the best known series is the 7.5-minute, 1:24,000 scale quadrangle, a non-metric scale virtually unique to the United States. Each of these maps covers an area bounded by two lines of latitude and two lines of longitude spaced 7.5 minutes apart. These maps are one-quarter of the older 15-minute series, hence the use of the word “quadrangle” to describe them. Nearly 57,000 individual sheets cover the lower 48 states, Hawaii, US territories and parts of southern Alaska. It is an absolutely huge number of maps, for example the entire of Great Britain is covered by only 403 maps of the Explorer series. That explains why the last of the quadrangles were finished only in 1992.

This is a vast and fantastic resource for any map geek like me. Even better, they are absolutely free to download and use. You can download scans of the original paper maps but the drawback of that is the fact that some of them are over 40 years old. Not really useful, especially for the fast changing landscape around the metropolitan areas.Wheeler Peak Edited

Luckily in the last few years (since 2009 to be precise) the USGS has produced a new series called US Topo. These maps are mass-produced using automated and semi-automated processes, with cartographic content supplied from the National GIS Database. They use the same familiar 7.5-minute format like the old paper maps but they are frequently updated and available in PDF format with geospatial extensions. The US Topo series is a really cool tool as you can switch on and off different layers of the map, like topography, or orthophoto image (aerial photography). Free tools also allow you to determine coordinates, measure distances and angles between points, measure areas, track positions using a GPS device, display coordinates in various map projections and print the map image. Some say that because these maps are produced automatically, based on various databases, and not field checked by humans like the old maps, they might be a bit less accurate, missing some information like for example some windmills or pipelines. On the other hand they are produced cheaply and quickly (at a rate of about 80 a day) and USGS aims to update each one of them at least every three years, with new information being added constantly into the database. Also, the US Topo maps all show Universal Transverse Mercator (UTM) grid lines while only about 25% of the old paper quadrangles do so.

Eureka NV Top

One major drawback of both the old paper quadrangles and the new US Topo maps is the fact that the individual sheets cover a very small area, only between 49 and 70 square miles (depending on the latitude).

Things get a bit complicated in Alaska. As I mentioned already, only some more populated parts of it are covered by the 7.5-minute series. The 15-minute series, at a scale of 1:63,360 (one inch representing one mile), remains the primary topographic quadrangle for the state of Alaska (and only for that particular state). Nearly 3000 maps (2920 to be precise) cover 97% of the state. These maps are often very old but there is nothing really better available for many parts of Alaska as the new US Topo series don’t cover this vast and still wild state. This should change soon as USGS is planning to start producing US Topo maps of Alaska some time this year. They will use unusual (for Americans) scale of 1:25,000.

The next interesting series is the 1:100,000 scale which cover an area of 30 minutes in latitude and 60 minutes in longitude. It is unusual in that it employs the metric system primarily, so contour intervals, spot elevations, and horizontal distances are all specified in meters, which may be a great advantage for Europeans. They also cover significantly larger areas than quadrangles, each sheet covering an area equivalent to 32 maps in the 1:24,000 series. However these maps are ageing fast, most of them are from the 1980s and the early 1990s.USGS 100k scale Porcupine Mountains

Apart from the current maps historical maps are also available to download from the USGS website. Currently about 141,000 out of the collection of the approximate 180,000 individual maps are available in PDF format.

So, this is USGS mapping in a nutshell, next time I’ll write more about other American map publishers.