Mapping, GPS, and GIS

 

 Mapping is an essential skill for archaeologists. Archaeology is the study of artifacts within a geographic context, and maps are necessary to provide that context. In the academic world, archaeologists use geographic data to study ancient settlement patterns across the landscape, and theorize about what those patterns might tell us about environmental adaptations, economic relations, or political power.

Mapping also has more practical applications for archaeologists within the CRM (cultural resources management) industry. The purpose of CRM is mainly to locate archaeological sites before they can be damaged by construction. Thus, accurately recording the locations of these sites is one of the most important duties of any CRM archaeologist. You cannot protect an archaeological site from an approaching pipeline or transmission line if you don’t know exactly where the site is.

 

Collecting Geospatial Data in the Field

In the early days of CRM archaeology—before the proliferation of GPS technology—mapping was a challenge. Archaeologists are generally not trained as land surveyors, and they often conduct pedestrian surveys in remote back country wilderness areas. In the old days of CRM, field archaeologists would trek out onto the open rangelands equipped with a compass and topographic map, and use their orienteering skills to plot newly found archaeological sites on their maps by hand.

This was not very precise or accurate. I was not a field archaeologist in the old days, as I did not enter CRM until 2011. But I’ve georeferenced* quite a few archaeological survey maps and site maps from the period prior to the year 2000, back when sites were often plotted by hand on USGS (U.S. Geological Survey) topographic maps. And in some cases, I’ve been able to compare these early maps with newer, more accurate maps that show the precise locations of these sites. On the old maps, archaeological sites are usually plotted in the wrong place, often by quite a wide margin. A good field archaeologist could plot a site on a map within about 40 meters of its correct location, but the plotting of some sites was off by more than 100 meters.

I should add that most of the maps I’ve personally georeferenced were drawn in northern Nevada, which is not an easy place to keep your bearings without modern GPS equipment. Northern Nevada is a sparsely populated desert covered in wide open rangeland, with many areas lacking distinct landforms that might be visible on a standard topographic map. Under these conditions, accurately plotting an archaeological site with no more than a topo map and a compass is extremely difficult. It’s a testament to the skill of the early field archaeologists that they were able to plot sites within about 40 meters of their correct locations.

But today, that level of accuracy is not quite good enough. A field archaeologist who accidentally plots an archaeological site 40 to 100 meters away from its correct location will end up protecting the wrong piece of land from construction, while inadvertently routing a pipeline or other construction project directly through the actual site.

Fortunately, the advent of GPS technology has added both accuracy and precision to our work. When the CRM industry began with the passage of the NHPA in 1966, GPS technology had not even been invented yet. But today, it is an indispensable tool for every archaeologist working in NHPA compliance.

The United States military invented GPS (Global Positioning System) in 1973, though it was not really operational until 1978. The military launched several navigational satellites into orbit, and these satellites now send signals to receivers back on earth. Trilateration of these signals allows a receiver to calculate its exact position on the earth’s surface. And thus, GPS was born.**

GPS technology was not available for civilian use until 1983, and even then, civilian access was restricted by the military’s policy of “selective availablity.” The military scrambled the signals being sent to civilian GPS receivers, ensuring that these receivers could not be accurate within less than 100 meters of the correct location. Selective availability remained in effect until Bill Clinton signed it away in 2000.

After the year 2000, now that civilians had access to much more accurate receivers, civilian use of GPS technology exploded. It became a handy tool for archaeologists and nearly everyone else. I was about eleven at the time, and I remember that this was the period when people who were much wealthier than me began to buy GPS navigation devices for their vehicles.

Twenty years later, the current archaeologist has access to a wide arsenal of GPS technology that can be used to record the real world coordinates of artifacts in the field, and upload those coordinates to a cartographic software program on a computer. The most popular devices (among archaeologists) have traditionally been made by the rival GPS companies Trimble and Garmin, but many archaeologists now use the internal GPS receivers found within their smart phones or tablets.

Garmin produces handheld GPS receivers that are accurate within about 3.65 meters of the real location under ideal circumstances (not accounting for clouds or trees that might obstruct the satellite signals). These devices are widely used in the Western states.

Trimble produces even more accurate devices. Any handheld device within the Trimble GeoExplorer series can be accurate within about 60 cm., or a little over half a meter. These are the devices that I have typically used during my career. A Trimble Juno receiver is accurate within about five meters, making it less popular among archaeologists.

Figure 1. Trimble GeoExplorer GeoXT GPS receiver displaying UTM coordinates collected in the field. This GPS receiver is capable of submeter accuracy.

All of the devices I’ve described so far are handheld devices that can be used on their own, but some companies produce special GPS receivers that can be paired with your personal smart device, such as a smart phone or tablet. The Trimble R1 and Juniper Geode are small GPS receivers that can pair with your smart phone or tablet via Bluetooth, offering submeter accuracy (much like a Trimble GeoExplorer). The internal GPS receiver inside your phone or tablet can't achieve submeter accuracy on its own. When used without an external receiver, your phone's GPS probably won't be accurate within less than about four meters (I've used a special app to test the accuracy of my field phone, and it was consistently accurate within about four meters—or so the app claimed).

If you don't need submeter accuracy, you can use a smart device on its own. A variety of data collection apps can be downloaded on any phone or tablet. One of the most well known is Avenza, but it is not the only one. ESRI has produced a geospatial data collection app known as Field Maps to replace its old app, known as Collector. 

All these devices have strengths and drawbacks. Any handheld device within the Trimble GeoExplorer series should be rugged enough to withstand the rigors of the field, while consistently providing submeter accuracy, but the data you collect is worthless unless it can be exported to a usable file on your computer. At the moment, the only way to do so is with a software called Pathfinder Office, which is very expensive. Garmin handheld devices are also very rugged, and it is cheap and easy to export data from them, but they are not very accurate (by archaeological standards). Smart phones and tablets are neither accurate nor rugged; you'll be lucky to achieve accuracy within four meters, and they are delicate little objects prone to breaking in the field. But you can use them to download a variety of different data collection apps to suit your needs. You can also pair them with a Juniper Geode or Trimble R1 to achieve high accuracy. These external GPS receivers are rugged, but the smart phones and tablets are not, and you can't really use the external receivers without a functioning smart device. Also, it is easy to lose the Bluetooth connection between smart device and external receiver without noticing, and then you end up using your smart device's internal GPS receiver to collect less accurate data than you need.

Also keep in mind that GPS receivers only work when they receive a signal from multiple satellites, and they aren’t effective if something obstructs those signals. Clouds and heavy tree cover can obstruct satellite signals. I once had to climb a tree in the field to get adequate satellite reception.

You can use any of this GPS equipment to collect real world coordinates in the field and upload those coordinates to a cartographic software platform on your computer. The study and usage of cartographic software is known as GIS (Geographic Information Science or Geographic Information Systems). For archaeologists, the most popular GIS software is ArcGIS, which is produced by a company known as ESRI. ArcGIS was originally designed for environmental scientists, but archaeologists frequently use it to process geospatial data and make maps. I was once a GIS technician specializing solely in archaeological data, and I used ArcGIS almost every day.

This is only a basic overview, because the process of uploading data and converting files can be complicated, and this is not meant to be a software tutorial.

 

Entering Geospatial Information Into Official Records

Now that you have a rough idea of how archaeologists record the locations of sites, it might be useful to know how those locations are entered into official records.

Every state in the United States is responsible for keeping records of the archaeological sites that are discovered during CRM surveys. Many states maintain online geospatial databases, where qualified archaeologists can log in and see where all the previously recorded sites have been plotted, as well as which areas have already been surveyed. For example, the state of Nevada maintains a geospatial database known as NVCRIS (Nevada Cultural Resource Information System), and when I was a GIS technician my job was to make corrections to the site boundaries and survey area boundaries within the database. Databases such as NVCRIS are restricted to qualified archaeologists. They are not open to the general public, out of the fear that members of the public might use this information to locate archaeological sites themselves and loot them for valuable artifacts. Looters commonly vandalize rockshelters or Native American graves in pursuit of artifacts that they can sell to collectors.

Before the introduction of geospatial databases, the individual states kept collections of paper maps on which the site boundaries had been drawn by hand. The state of Oklahoma (where I did fieldwork for my Master's thesis) did not have a geospatial database for archaeological sites until the end of 2022. Before then, anyone who wanted to know which sites had already been recorded in Oklahoma had to go to Norman, Oklahoma in person, and go through a collection of topo maps that had archaeological site boundaries drawn over them in pencil.

If you find an archaeological site in the field, you will need to submit your findings to the appropriate state government, and sometimes to a federal agency. Typically, this entails that the site boundary be plotted on a USGS topographic map. Originally, the site boundary would be drawn on the map by hand. Today, this can be done digitally, with ArcGIS or some other computer program. You may also be able to submit your geospatial data directly to the government agency, in the form of an ESRI shapefile.

 

Legal Descriptions

You will also need to fill out a site form. Different states use different formats for their site forms, but any state that uses PLSS (Public Land Survey System) will require geographic information in the form of a legal description.***

A legal description is what land surveyors use to keep track of parcels in their official records. In order to explain how to write a legal description for an archaeological site, I’m going to show you how to write the legal for the abandoned brick grain silo outside the house where I grew up in rural Illinois. I can’t show you the location of a real archaeological site that has been recorded in a database, because that information is confidential. As I’ve already explained, the locations of archaeological sites are not made available to the general public, due to the prevalence of looting. But there’s probably no harm in showing you the location of the old silo outside my dad’s former home (even though it could technically be considered a historical archaeological site, as it was built around the year 1900). It contains nothing of monetary value, and if you feel an urge to trespass there, be aware that the current landowners probably have a lot of guns, as is customary in rural Illinois.

Here is a satellite image of the old silo:

 

Figure 2. View of historical silo from space

Land surveyors in the United States write legal descriptions using a system known as PLSS (Public Land Survey System). I’ve noticed that even experienced archaeologists have trouble understanding this system, so I wanted to devote some special attention to it. The best way to explain PLSS is to start with the biggest units of land, and work our way down to the smaller divisions.

The silo outside my old home is located in Peoria County, Illinois, as shown below:

 

Figure 3. Map of historical silo's location imposed over Illinois counties

The state of Illinois is further divided by township. Essentially, it is divided into a set of perfect township squares, with each square covering 36 square miles (6 miles by 6 miles on each side). Each one of these squares is assigned two numbers—one for “township” and one for “range” (but the individual squares are generally just called townships). The "township" number indicates how far north or south the square is, and the "range" number indicates how far east or west it is. The township number is counted from an imaginary line called a baseline, which runs from east to west (for example, if the township unit is labeled as 9 N, that means its northern boundary is 54 miles north of the baseline). The range number is counted from an imaginary line called a principal meridian (for example, if the unit is labeled as 5 E, that means its eastern boundary is 30 miles east of the principal meridian).

That is how the term “township” is used in PLSS. It has a slightly different meaning in local politics. A “township” can also refer to an administrative division of a county, with its own small local government (road commissioner, accountant, etc.). These political units usually correspond perfectly with the 36-square-mile township squares I’ve discussed above. In some cases, the political boundaries of a township may be re-drawn to make governance easier, but the PLSS units are not supposed to change.

For example, I grew up in a remote area of Elmwood Township in Peoria County, Illinois. “Elmwood Township” is a political/administrative unit. It also happens to correspond with a 36-square-mile PLSS township unit located 48 miles north of the baseline and 24 miles east of the fourth principal meridian. Thus, the legal description so far would read as follows:

Township: 9 N

Range: 5 E

Principal meridian: 4th

Figure 4. Map showing silo's location and PLSS township boundaries in relation to baseline and principal meridian. The silo is located in the 9th township north of the baseline (hence, Township: 9 N) and the 5th township east of the principal meridian (hence, Range: 5 E). Note that the townships east of the Illinois River are counted from a different principal meridian, not shown in this map.

 

Figure 5. Map of silo's location imposed over PLSS township boundaries within Peoria County, IL. PLSS townships fit within county boundaries, but the county is not really part of the legal description.

Now, let’s work our way down to even smaller PLSS units. Each township/range square is further subdivided into 36 sections, with each section equaling one square mile. Section 1 is in the northeast corner, and the individual sections are numbered in alternating rows, ending at Section 36 in the southeast corner.

 

Figure 6. Map of silo's location imposed over section boundaries within Township 9 N Range 5 E

For example, I grew up in Section 23, meaning that the legal description so far is:

Section: 23

Township: 9 N

Range: 5 E

Principal meridian: 4th

Figure 7. Map of silo's location imposed over Section 23

In order to write an archaeological site’s legal description with the level of precision required by a state government, you generally need to divide the section into quarter sections three times. You will be narrowing down the site’s location to a square patch of land covering only 1/64 of a square mile. That is how precise your legal description needs to be.

First, I need to divide Section 23 into quarter sections: NW, NE, SW, and SE. The silo is located in the NE quarter. Then I divide the NE quarter into four quarters. The silo is in the SE quarter of the NE quarter. Finally, I divide that quarter into four more quarters. The silo is located in the SE quarter of the SE quarter of the NE quarter of Section 23. Thus, the full legal description reads as follows:

Quarter sections: SE SE NE****

Section: 23

Township: 9 N

Range: 5 E

Principal meridian: 4th

In all likelihood, you will need to do this for every site you record during a survey, and write the full legal description on the site form (Site forms generally do not require that you list the principal meridian, but you will have to name the county, and that should narrow down the location sufficiently).

UTM Coordinates

You will also need to learn a little about the UTM (Universal Transverse Mercator) grid system. This is a navigational coordinate system. When you use your GPS equipment to record the real world coordinates of artifacts in the field, you will often need to use UTM coordinates (as opposed to Latitude/Longitude coordinates, which belong to a different coordinate system).*****

Under the UTM grid system, the earth is divided into longitudinal zones. For example, the silo outside my old house is located in UTM Zone 16 North, which would be written as 16T. The precise UTM coordinates of the silo would be written as follows:

16T 255983 4514944

The first number (16T) indicates the UTM zone.

The second number (255983) indicates Easting.

The third number (4514944) indicates Northing.

It will be pertinent to know that the UTM grid system changed in 1983. The original system is known as NAD 1927 (North American Datum 1927). For some complicated geodetic reasons that I don’t fully understand myself, the 1927 system proved inadequate when being used with GPS technology, so the datum (frame of reference) was shifted, and the whole grid shifted with it. The new system is known as NAD 1983 (North American Datum 1983), and it was implemented due to the increasing usage of GPS technology at the time.

Why does this matter for archaeologists? Because the UTM coordinates written on old site forms often still use the 1927 system. In fact, archaeologists were still using the 1927 system until well after 2000. Old copies of USGS topographic maps use the 1927 system as well.

If I use the NAD 1927 grid system to determine the UTM coordinates of the old silo, I get the following coordinates:

16T 255976 4514736

Keep in mind that the silo itself has not actually moved. But there are two UTM grid systems, and they will give two completely different sets of coordinates to the same geographic location.

This issue is more likely to come up than you might think. There may be previously recorded sites within your survey area, and you will need to re-visit them during the course of your survey, possibly with the aid of the original site forms. But the UTM coordinates written on the site form may belong to the NAD 1927 system, and if you try to navigate to those same coordinates using the current NAD 1983 system, the coordinates will lead you to a completely different spot (about 200 meters south of the actual location). If you are reading or writing UTM coordinates, always be sure you know whether they belong to NAD 1927 or NAD 1983.

Unfortunately, many archaeologists during the old days were really bad at recording UTM coordinates, so the UTMs written on old site forms are often completely wrong, no matter which grid was being used. If you want to use a site form to locate a previously recorded site, but the UTMs on the site form are wrong, the best solution is to look for the physical features of the landscape that are drawn on the sketch map, which should be attached to the site form.

Quadrangles

Site forms generally require that you specify which "quadrangle" the site falls within. Personally, I don't think this is particularly useful information; this is mainly a holdover from the days when archaeologists plotted sites by hand on USGS topographic maps.

Quadrangles have nothing to do with PLSS units, and this can confuse archaeologists, so I'm going to break down what a quadrangle is.

A long time ago, the U.S. Geological Survey began producing topographic maps for the entirety of the United States. They created sets of maps at different scales, but archaeologists (and most other people) took to using maps made at the 1:24,000 scale (meaning that a unit on the map is 1/24,000 the size of its corresponding unit in real life). The USGS maps made at the 1:24,000 scale are known as 7.5 minute maps, because they cover a rectangular area corresponding to 7.5 minutes (a minute is 1/60 of a degree, and the globe is divided into 360 degrees). The rectangular area covered by a single 7.5 minute map is known as a 7.5 minute quadrangle.****** In effect, the USGS divided the whole United States into about 57,000 quadrangles, and named each one of them (they also created quadrangles at different sizes for maps made at different scales, but I'm not going to get into that, because archaeologists don't really use maps at the other scales).

These USGS quadrangles are perfect rectangles that cross state and county lines; they have nothing to do with the boundaries used by land surveyors or government administrators. However, the topographic maps themselves typically show PLSS units and UTM grid lines (be aware that maps made before 1983 will use the old UTM grid from 1927).

The historic silo I've been discussing falls within a 7.5 minute quadrangle known as Farmington East Quadrangle (named after the nearby town of Farmington, Illinois). The map below shows its location, imposed over the dozens of quadrangles that cover parts of the state of Illinois (note: this map does not show all quadrangles in the United States).

Figure 8. Location of historic silo imposed over 7.5 minute USGS quadrangle boundaries. The USGS has created a 1:24,000 scale topographic map for each of the rectangular units shown in the above map. The silo is located at the northern end of Farmington East Quadrangle.

Overview

This is the bare minimum of mapping knowledge necessary for conducting surveys and recording your findings. In fact, it’s less than the bare minimum, because I didn’t teach you how to use the various types of hardware or software. Ideally, if you want to make maps, you should also know a little about the principles of cartography, such as normalizing data, and knowing which map projections are appropriate for presenting which kinds of data. But I don’t have time for that here.

GIS and cartography have purely academic applications as well, both within and outside archaeology. I’m not going to discuss that here, but for those who are interested, there are plenty of studies about landscape archaeology and settlement patterns.

Footnotes:

*"Georeferencing" is the process of assigning geospatial coordinates to specific points in a pixelated image, such as a scan of a paper map.

**The term "GPS" refers specifically to the satellite system put into orbit by the United States military. Other countries have launched navigational satellites as well. The systems that include these satellites are known collectively as GNSS (Global Navigation Satellite System). Many of the devices that we colloquially call "GPS receivers" (and which I refer to as GPS receivers in this post) can also serve as GNSS receivers, if they are allowed to access international satellite signals.

***Not all states use PLSS, but if you are working in states that do, reading a PLSS legal description is an essential skill for archaeologists. Some states that used to be part of Mexico (such as Texas) do not use PLSS (for example, Nevada and California both use PLSS, and thus their site forms require legal descriptions, but Texas does not use PLSS, and thus does not include legal descriptions in its site forms).

****When I determined the quarter sections for this silo, I used the section boundaries on a USGS topographic map, not the Tigerline shapefile shown in the map I produced for this blog. The Tigerline shapefiles do not match up perfectly with the section boundaries drawn on USGS topo maps, but the topo maps are probably more accurate.

*****The numbers in a set of UTM coordinates represent meters on the earth's surface. For example, "northing" indicates the number of meters north of the equator. "Easting" is more complicated, which is why you also need to include the UTM zone in a set of coordinates. The Latitude/Longitude coordinate system is completely different—it divides the round globe into degrees, minutes, and seconds (in the same way that you can divide a circle into 360 degrees in a geometry class).

******The actual area that a 7.5 minute quadrangle map covers will vary by latitude. USGS topo maps use a conformal projection, meaning they preserve angular integrity (you can use these maps to follow a compass bearing), but in doing so, they must sacrifice areal accuracy (they do not show units as being the correct area, and this discrepancy becomes more pronounced at those latitudes closer to the poles). This is one of the complexities of trying to represent a round planet as a flat surface.

Other Resources

The sources below may be useful for students wanting to learn more about the academic applications of mapping in archaeology.

Barrett, J.C. and I. Ko. 2009 A phenomenology of landscape: a crisis in British landscape archaeology? Journal of Social Archaeology 9:275-294.

Hally, David J. 1993 The Territorial Size of Mississippian Chiefdoms. Archaeological Report 25:143-168.

White, Devin A. and Sarah L. Surface-Evans. 2012 Least Cost Analysis of Social Landscapes: Archaeological Case Studies. University of Utah Press, Salt Lake City.