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Mapping the Pratt Mammoth excavation using GPS and basic surveying technology

The discovery of a partial mammoth skeleton in 1999, and its subsequent excavation in 2000, provided an opportunity to implement several innovations in the on-site mapping of the excavation and relating the excavation to real-world coordinate systems. What follows is a basic primer on what we did.

The mammoth specimen was found during the excavation of a waste-water lagoon on property owned by the City of Pratt, Kansas being leased to Pratt Feeders. While digging the lagoon the heavy equipment operator encountered a hard lens of sediment. Upon digging into it, several large bones were found. News of the discovery found its way to a reporter at the Pratt newspaper, who subsequently contacted me, then at the Sternberg Museum of Natural History. A small group from the museum traveled to Pratt for an initial investigation of the site. After an afternoon of excavating around the exposed bones it became clear that the site was more extensive and a longer excavation was needed. We planned to return to the site several weeks later.

A four-day excavation was planned with numerous volunteers and we returned to the site in November, 1999. After this exploration it again was clear that a longer period of time was needed to fully investigate the site, as we kept finding more and more fossil bone. We re-covered the site once again, and planned to return in the summer of 2000.

A volunteer excavates around a mammoth vertebra at the Pratt Mammoth site.

A volunteer excavates around a mammoth vertebra at the Pratt Mammoth site.

Because we had time to plan a larger excavation for the summer, and it was clear that there were many bone elements preserved, I wanted to be sure to map the site in detail, both for its paleontological resources, but also for other physical characteristics. So, I arranged to have a surveying total station on hand for the dig. We also lined up numerous volunteers, arranged a university class to be taught using the site as a learning tool, and obtained numerous donations from the generous community of Pratt. The entire community got behind the excitement of the dig, and because the site was easily accessible being at the airport, we hosted numerous visitors.

Establishment of points within the site

During the November, 1999 dig, I had established three control monuments at the site. The monuments were a hub (2 inch x 2 inch stake) and tack (a special surveying nail) driven into the ground away from the areas we were going to be disturbing in our excavations. This insured that we could re-find the control monuments and they would be used to relate all the points of the excavation to each other within the dig area.

Using the control monuments, we established an arbitrarily-oriented meter grid system. I was not concerned with the cardinal orientation of the grid so much as wanting the grid to be useful for in-site control of the dig. One of the principle uses of the grid was to demarcate 2 x 2 meter spaces to assign to volunteers to control their digging efforts. Having the grid on the ground helped to keep their efforts orderly, as they could be assigned to “dig here” and not be on top of each other. The use of the surveying total station allowed for accurate layout of the grid system across the entire site, and allowed for unlimited expansion of the system as needed.
 
The reason that the grid was redundant for the within-site location of bones is that all bones were located with the total station using standard radial surveying techniques. Every element removed from the site was given a field number, and the location of the element was documented in three-dimensions. Initially, the grid system was assigned assumed x and y coordinates in meters, and an assumed elevation was assigned to the control monuments so that the z dimension could be calculated relative to other points in the site.

The total station is set up over a point and aliened with another control point to get a starting line. The instrument can accurately measure distances by shooting a laser to a reflecting prism and measuring the time it takes to return to the instrument. It also accurately measures horizontal and vertical angles. With the vertical angle and the distance, it can calculate the difference in height (z) between the reflector and the instrument. So, with relatively simple calculations the x, y, and z coordinates of any point within the site can be determined. The instrument is highly accurate (within 1/1000’s of a meter in distance) so within-site accuracy is estimated to be high, likely within a centimeter or two given the reliability of using inexperienced volunteers to help with the surveying.

Teaching a young volunteer to use the total station surveying instrument.

Teaching a young volunteer to use the total station surveying instrument.

Most of the elements removed from the site were located with a minimum of two points. The smallest bone fragments were located with point locations of a single measurement. If they bone had any linearity to it, it was located as two points (end and end) giving both the approximate length of the bone and its linear orientation. Several of the larger bones were located with three or more points.

All of the points located at the site are described as their three dimensional coordinates, and therefore can be plotted for visualization.

Point cloud from the Pratt Mammoth site shown in map view with the meter grid.

Point cloud from the Pratt Mammoth site shown in map view with the meter grid.

View the point cloud in a short animation.

Translation and rotation to real-world coordinate systems

During the excavation we used assumed coordinates and elevations. However, it is desirable to be able to locate the site, and all the points located within the site, in a real-world coordinate system so that it can be related to other localities anywhere in the world. We did not have high-precision global positioning system (GPS) equipment available, although such equipment does exist. However, using the following method we were able to get very effective results using a basic Garmin handheld GPS unit.

The accuracy of the handheld unit varies with availability of satellites, access to the open sky, variations in atmospheric conditions, basic limitations of the unit itself, and other variables. However, it is possible to locate a point on the globe to within approximately 15 feet or so. I used the GPS to record the location of two of my control monuments. I used the coordinates of the GPS reading to calculate the azimuth between the control monuments, and assumed that was the true azimuth. I knew the distance between the monuments based upon my field survey of measuring between them. I assumed the GPS reading on one of the control monuments to be true and “held” its coordinates to that reading. Using the azimuth to the other monument from the GPS reading, and the distance measured in the field, I then calculated the new coordinates for the second control monument.

With the assumption of the coordinates of the first monument, the accurate real-world azimuth, and the measured distance to the second control monument providing its coordinates, it was possible to recalculate the coordinates of every point within the dig site. It is a basic mathematic routine to translate (move points horizontally in space) and rotate (turn the points on an axis in space) all the points located at the dig site to a very close approximation of their real-world coordinates. Since we measured all the points in meters I used the Universe Transverse Mercator (UTM) coordinate system. I estimate that the accuracy of these coordinates should be within about 15 feet (the error of the reading on the handheld GPS). This is not as accurate as you could get with high-precision equipment, but it is accurate enough for almost all purposes, and can be achieved with inexpensive equipment that is readily available.

I have modified the system somewhat, but I have since used this basic system at other excavation sites with very good results. The real-world coordinates of every point from the site allows very accurate plotting of fossil sites, and even individual bone elements, in relation to other sites. It also allows for the application of geographic information systems (GIS) technology on the sites.

Bones drawn using the points located at the Pratt Mammoth site.

Bones drawn using the points located at the Pratt Mammoth site.

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UTM

In a couple of previous posts we have examined latitude and longitude in some detail, and explored in what format the numbers might be displayed on your handheld GPS unit. Here we will explore another commonly used coordinate system: UTM.

Latitude and longitude work well, but since they are all based upon circles, and because degrees are divided into groups of 60, the calculations required to work with them are unwieldy. It is frequently useful to have coordinates based upon plane rectangular geometry in our normal base 10 system. This can work well if the area in question is not too large, because then we can discount the curvature of the Earth. Over just a section of the Earth, the curve does not make a huge difference in error.

In order to do this, we must “flatten” the Earth to a plane, and when we do we can use the Cartesian coordinates that we all used in high school geometry class. It would be like flattening the peel of an orange. If you try to flatten the whole peel it makes a very irregular shape. But if you flatten a small piece it does fine. Then you can think of the x and y axes as east-west and north-south.

Cartesian coordinate system illustrated

The familiar Cartesian coordinate system showing how points can be assigned a distance along two axes from the origin point.

As you recall, every point on this plane can be described by two coordinates, one along each axis. And you can see in the  illustration that some of the points have negative values with respect to the origin (point 0,0). However, we can control how we place the coordinate system, and with the origin point outside of our area of interest all the points can be positive (east, north)—in the upper right-hand quadrant.

State of Kansas laid on top of a Cartesian coordinate form

The State of Kansas placed on the rectangular grid so that every point in the state will be positive with regard to the origin.

UTM coordinates, or Universal Transverse Mercator coordinates, are a rectangular plane system in metric units. Cartographers have taken swaths of each state and mathematically flattened them into a plane, assigned an origin southwest of each section, and laid out the resulting grid. Some of the smaller states are covered in a single swath, but the larger states must be broken into several zones so that the curvature of the Earth does not distort each map too much.

So, UTM coordinates have two components: their easting and northing values from the origin point in meters, and which state zone you are referring to. You can see a map of the zones here.

I have used UTM coordinates to good effect in my scientific work. When mapping a paleontological excavation, for example, we make all measurements in metric units. Since we are already measuring in metric units, every point in the dig site can easily be assigned its real-world UTM coordinate, instantly relating all points in the dig to any other point on the globe.

This means we can accurately and quickly plot the location of any fossil site, or even individual fossils, on a real-world map.

Related posts:
Recommended handheld GPS units
handheld GPS basics
Basic features in a handheld GPS
Geocaching

Mapping the Pratt Mammoth

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Latitude and longitude 2

In the first discussion of latitude and longitude, we investigated how the latitude-longitude grid was established. In this post we will look at how that relates to the display on your handheld GPS unit.

If I stand outside with my GPS and direct the unit to display my position, it may do so in a couple of different ways. First, it might show my position on a map background, pinpointing my location with regard to other features such as streets, buildings, or landforms. This shows me my location as if I were walking around on the map.

I might want to know my latitude and longitude coordinates. Perhaps I want to record them so I can return to this spot in the future. (You can understand why a “bone digger” would want that!) In most handheld GPS units you can save your location as a waypoint. (See my review of the best GPS units.)

A really useful thing to be aware of about has to do with the format of latitude and longitude number displays. These numbers can be expressed in several ways, and you must know which format the numbers are in or risk making errors. After GPS units first came out, I used one to locate some fossil sites where I collected. I happily recorded the coordinates for the fossils I picked up, and dutifully wrote them in my field book. Only later did I realize that I was not perfectly clear which form the coordinates were in, making the records almost useless!

Most handheld GPS units allow you to display the coordinates in several formats. You could show the form degrees-minutes-seconds, sometimes denoted as DMS. This might look something like 38°53’22.49″N, 99°17’58.73″W. Latitude is given as degrees north or south from the equator, and longitude is given as degrees east or west from the prime meridian.

But we could also give these same numbers in another format which would look like: 38° 53.375’N, 99° 17.979’W. This format is in the form of degrees, minutes, and decimal minutes, or DMM.

Finally, we could show these coordinates as full decimal degrees (DDD) and it would look like: 38.889583°, -99.299650°. Note that in this form, the positive or negative form of the number is important as that gives the direction. Positive latitude numbers are north, and negative longitude numbers are west.

Notice that these forms are all equal: 1° 30′ 30″ (DMS); 1° 30.5′ (DMM); 1.5083° (DDD). And you can see if you just wrote down numbers on a page, and were not very clear about which form the numbers were in, you could be very far off the mark in terms of location.

There is another common coordinate form called UTM which we will examine in another post. Bonus points to anyone who can tell me what is at the coordinates used in this post.

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Geocaching

Do you enjoy hiking and exploring the outdoors, seeing new places, and having fun? Maybe you want to try geocaching.

Geocaching (pronounced geo-cashing) has fast become a popular pastime for outdoor adventurers, especially those who use a handheld GPS. Basically, geocaching is a grown-up version of hide and seek. Someone places a “surprise” package, or cache, and provides its latitude and longitude coordinates to others who then try and find it again.

This activity is enjoyed by people from all walks of life, young and old, and is great fun. There is a hint of the exotic to the hobby, making you feel like Indiana Jones seeking out lost and hidden treasure.

The cache is usually stored in a water-tight container to protect it from the elements, and often contains a log for visitors to sign. Sometimes there are trinkets to be found too. Caching etiquette dictates that you are welcome to take one of the trinkets, but you must leave something in return.

Maybe the cache hider will leave a disposable camera for example, and request that visitors take a picture of themselves and return the camera to the box. This way there is a fun record of who all has braved the elements and sleuthed out the cache’s location.

You can find the locations of geocaches near you, or near where you want to explore, through web sites like geocaching.com. A basic membership there is free and gives you access to the locations of geocaches and allows you to share your adventures with others.

If you really get into this hobby, maybe you would like to create a geocache for others to find. There are guidelines that should be followed as the goal is to have a safe and fun experience while protecting the environment around the cache. First, be sure you have the permission of the landowner or manager. If people are going to be visiting the area looking for your geocache, they don’t want to be chased off by a surprised and angry landowner.

You need to hide your cache were it is unlikely to be found by a casual visitor, and in such a way that getting to it will not harm the local resources. You should leave a log and a writing utensil, preferably a soft lead pencil if the area will get below freezing. Also, leave a note for the hunters, explaining why the location is important to you, and explaining the basic idea of the game for someone who does stumble upon the site accidentally. Obviously, do not leave food or anything dangerous or illegal.

A good handheld GPS is critical for this activity as it is based upon recovering the coordinates of the geocache. You can find information about GPS units, basics of their features, and recommendations on models to purchase here at boneblogger.

Check back here often because we will be posting more about this fun outdoor activity. Have fun, and be safe everyone.

Related posts:
Latidude and longitude
Latitude and longitude 2
UTM

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Basic features in a handheld GPS

In the basic post on GPS, we explored what GPS was and urged that you consider your needs for a handheld GPS before buying one. There are many features available on the various models out there, and there may be some features you would like, and likely some you would never use.

Screens

But before we get into the more exotic features, let’s look at the basics. One of the most important and basic features of any GPS is its screen. The screens on handheld units are basically of two flavors—monochrome and full color. As you would expect, the color screens are a premium feature and add to the cost. Screen size is also a significant consideration. A small screen can be very hard to see, and even if it looks OK in a store, taking it out into bright sunlight can really change a screen’s visibility.

Unit Size

Overall size and weight of the GPS unit are also something to consider. If you are planning to carry the unit around on hikes, small is definitely better. If it is so large and heavy that it gets left behind, it does you little good. It might not seem like it, but every ounce counts when lugging it around all day. On the other hand, if you are mounting it to your bike or boat, or driving into remote areas, its overall size might be less of a limiting factor.

Data handling

Be sure to review the unit’s ability to store waypoints and routes. A waypoint is simply the coordinates of any point you would like to reference again. For example, you might want to mark your car, camp, or house as a waypoint. It is fun when you are out in the boonies to see the direction and distance back to the house (“Look, we are 454 miles from home!”). Most units allow you to store several hundred to a thousand or more points.

A route is a series of connected waypoints that moves you from a beginning point to a destination. For example, on a hike you might start at the trail head, travel to a junction with another trail, take the right fork, and end at your camp. The route on the GPS will direct you from point to point as you go.

Some GPS units will allow you to save tracks. This is like leaving digital breadcrumbs. As you travel, the unit will mark your path along the way, showing where you have been.

Some of the GPS units allow you to connect them to a computer and move information back and forth between them. This is really handy in a number of ways. You could plan your route at home on the computer, upload the route to the GPS and let it take you on your way. Or, you might like to wander off and blaze your own trail, then download the track record to your computer to save a record of where you went.

Another important information-sharing feature is being able to upload additional maps into the handheld GPS unit. Most come with pre-installed maps, but depending on the unit, its maps might be more appropriate for a day hiking the golf course than trekking through seldom-explored wilderness. You can purchase software and map sets to load into your GPS so you could have full map details of the region you are planning to visit while in the field. You can also get free maps for Garmin GPS units at GPSFileDepot.

Weather resistance

Units might be billed as water proof or water resistant. Water resistant means that the unit can handle being splashed with water. Water proof means the unit can take being fully submerged in the creek or lake. Clearly, if you are going to be out in inclement weather, water proof is going to be an important feature. However, water proof does not mean you should plan to swim with it.

Signal receiving

Units vary as to how well they can receive the signals from the satellites. More expensive models often feature better signal capabilities that allows them to provide your location through trees and other sky-coverage. If you are going to be using your unit to travel in remote regions, make sure to get a high sensitivity receiver. You can also explore getting one with Wide Area Augmentation System (WAAS). This system supplements the standard GPS satellite system and provides for increased accuracy. It is not available everywhere, but if the unit you have is capable of receiving the WAAS signals, it will at no extra cost, and will increase the accuracy of your locations up to five times.

Battery life

Finally, you might consider battery life and type. Most units run on disposable batteries, typically AA, but they do not all consume batteries at the same rate. Some units have AC adapters, and some do not. Some have an internal rechargeable battery. Each style has trade offs. For example the disposable battery option might be fine for use a few times a year, but if you are a regular adventurer, you could spend a lot of money on batteries in a season.

Those are some of the basic features to consider. In future posts we will look at some of the fancier features, and investigate specific models.

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