MapHeads

How to be two places at once: map datums

So you have a fancy new GPS unit, satellites fixes streaming in, and not a tree or building to get in the way. Your GPS does it’s fancy black-box magic and *bang* you’ve got a lat/lon for the point on the earth right under your feet. Pretty cool.

Now you look up the coordinate on your map. The point turns out not to be under your feet, but 200 yards away in the neighbor’s swimming pool. Huh? How are your feet in one place and the same map location completely different? Most likely your GPS and map are using different map datums.

The Earth is round. Learned that one in kindergarten, right? Well, it’s not. Not precisely round, that is. The Earth is actually a somewhat elongated lumpy ball of rock. In order to translate a geographic coordinate (or a whole map) onto the surface of the lumpy ball, you need a map datum.

A datum basically defines the position of the Earth’s surface relative to the center of the Earth. Initially these datums and their underlying ellipsoid models (fancy mathematical models describing the shape of the earth) were established by series of stations surveyed out from a central control point. A common datum used in the United States is the North American Datum of 1927, or NAD27. Not so coincidentally most USGS topographic maps made before 1985 used the NAD27 for a base datum.

As satellite technology and observations became available in the 1960s and 1970s, new “global” mathematical models (and datums) were developed that began to supercede the earlier models. The World Geodetic System of 1984 (WGS84) is now the standard for GPS technology worldwide, and in North America the North American Datum of 1983 (NAD83) was created to replace the aging, and somewhat innacurate NAD27.

Any good map should tell you what datum it’s based upon. Since your GPS is actually a fancy little computer, you can set it to the same datum as your map. Most GPS units are pre-set to WGS84, which is for most practical purposes identical to NAD83.

When you order a map from MyTopo.com, you have the flexibility to choose traditional NAD27 or the newer NAD83, and the datum of your map is printed right in the legend.

Voila, you’re back where you started.

So where’d you get that topo map?

In a past installment we’ve provided a brief history on the the USGS topographic map. There’s a lot of products out there with topo maps: web sites, CD sets, paper maps. So where did they come from, and how can you tell the difference?

In the mid 1990’s the US Geological Survey began producing digital scans of the original paper topo maps. The scanning program was completed in the past few years, and the result is a widespread collection of digital topos that power everything from topo map websites such as Microsoft Terraserver and Topozone, to our own map printing service at MyTopo.

Not everybody uses the USGS scans (map software such as National Geographic Topo! And MapTech Terrain Navigator), and there are some good reasons why. One is that they were developed for digital use in mind, so they don’t work well for all purposes. We’ve had good look converting and enhancing the database to make great prints and digital products, and they provide a cost-effective map source that helps us keep our prices low. Perhaps more importantly, the USGS scanning program did not result in a complete set of digital maps. The Forest Service Single Edition maps we talked about in a previous article, for example, were largely left out of the scanned database.

To those of us that use the USGS digital maps, it became apparent over the past few years that if we wanted to provide the most current, accurate maps available we were going to need to take the situation into our own hands. The folks at Igage Mapping (producers of the AllTopo map software products) were on the ball and agressively filled the hole left in the USGS scanning program with a map updating program of their own. At MyTopo we were fortunate to tag along, and now have over 5,000 new digital maps that don’t exist from many other sources.

Many of these new maps are from the Single Edition series, and others are map updates that came after the now-expired map scanning program.

So there are lots of sources for topo maps out there, but they’re not always identical. We at MyTopo have tried to strike a middle ground, while making sure that we always provide the most updated maps availabe.

A quick history of the USGS topographic map

One of the most common and understandable questions we receive at MyTopo is “when do you update your maps?” Like most topographic map services, our maps are based on the US Geological Survey topo maps. So the quick answer is, whenever the USGS releases a new map! If we dig a little bit deeper, there's more to the story about where these maps come from and where they are going.

The USGS topographic map is a pretty amazing resource. From the 1940’s through 1992, the National Mapping Program (NMP) at the USGS pumped-out over 55,000 uniform, detailed topo maps. The flagship 7.5 minute map series, what we generally call “USGS Quads”, provides a base map that stretches seamlessly from coast-to-coast. These maps became the standard for what we in the United States think of as a “topo map”.

With the completion of the 7.5 minute topo map series in 1992, the USGS switched modes from new map production to map revision. To make a long story short, even what the USGS calls “Basic Revision” is not cheap: an average of $17,000 per map! With over 55,000 maps updating the USGS quads gets expensive fast

The revision programs of the 1990’s produced thousands of new map releases, but the Forest Service Single Edition maps provided the largest contribution of new topographic map information. Topo maps in most National Forest lands are a good deal more interesting than they were before this program: they’ve added road networks including Forest Service roads (with road numbers for easy identification in the field), recreation areas, campgrounds, and even outlined private land “inholdings” within the Forest boundaries so you have a better chance of staying on public lands in the woods.

The Forest Service Single Edition topos are not available most places on the Web, and the folks at Igage Mapping (producers of the AllTopo map software products) were on the ball and agressively filled the hole left in the USGS scanning program by aquiring and scanning all of these maps. They were good enough to share a copy of the new maps with us, and now we have added all of these published maps (and some others the USGS didn't scan) to our map database.

So we’re confident we have the most updated USGS topo maps available. What’s the future of topo maps and plans for updating the one’s already out there? The answer is a bit deeper and we’ll be back with more on the future of the topographic map.