Error processing SSI file
 
Technology

Technology FAQ
1. I want to use electronic calipers to do data entry. How is this done?
2. I want to buy a total station. What model should I buy?

See also our FAQ on the EDM software

Answers

1. Taking electronic caliper measurements is simple, fast and not terribly expensive. We use Mitutoyo calipers with a USB interface. With this equipment installed you can send measurements directly to any software including Excel and Access. It will work on both Mac and PC operating systems. We use our own data entry program that allows you to make custom data entry forms. This software is written for Windows.

2. Assuming you are not buying a high-end total station, there are four main decisions to make: do you want the option to shoot reflectorless, what is the furtherest distance you will be shooting (both reflectorless and with a reflector), what precision do you require, and how will you store the data points. As for precision and distance, fortunately for archaeology typically the least capable (ie. least expensive) machine is sufficient. For a long time we used 5 second machines (meaning that all angle measures are rounded to the nearest 5 seconds) and had no issues doing piece provenience work or making maps of the areas surrounding our sites. We have also worked with 3, 7 and even 10 second machines. The effect of inprecise angle measurements on the resulting XYZ coordinates is a function of distance. As can be seen from the table, over the distances that most archaeologists work, the effect is likely within a tolerable range. In this table, the error is the maximum error (ie. the angle is off by 1, 2, 5, or 10 seconds) and the horizontal error is listed in centimeters (note that there would be an additional, similar Z error).

1 second2 Seconds5 Seconds10 Seconds
10 Meters0.000.000.000.00
50 Meters0.000.000.100.20
100 Meters0.000.100.200.50
1000 Meters0.501.002.404.80
5000 Meters2.504.8012.2024.10

With regard to the distance that one can measure, this is in part a function of the prism size. A small prism will reflect less of the total station's signal and will be harder to see in the crosshairs of the instrument at greater distances. To piece provenience artifacts, we use the smallest prism available, but to survey larger distances you will want to purchase a larger prism. With a larger prism, most instruments can take a single measure across many hundreds of meters and typically close to a kilometer. Before you spend more to go beyond this, it is worth considering how often you will need to record these distances. A long distance can be easily traversed with multiple shorter measurements.

When we first started buying total stations, recording a point without a prism was not even possible. Now they make machines that do not even offer the option of recording with a prism. A fewer years ago whether to purchase a reflectorless option or not would have been a harder decision; now, unless you are considering buying a used instrument, you are likely going to purchase a machine that can record reflectorlessly. We recommend this. For piece proveniencing artifacts, we still use a mix of reflectorless and non-reflectorless recording, but reflectorless is extremely useful for making topographic maps of a site (especially for inaccessible cave walls and the like) and for doing initial setup using targets afixed to walls, buildings, cliff faces, etc. Importantly, if you think you will rely on reflectorless mode, one of the ways machines vary is in the distance they can record a reflectorless point and this distance can be far less than what can be done with a reflector. To record a point at a greater distance requires more power, and this will cost more. You will want to check the specifications carefully on this point.

Finally, one other major decision to make is how the data points will be stored. There are several ways to approach this. What many do is to store the points in the memory of the machine and to record separately (on paper or in Excel) the information for each point (eg. level, type of object, etc.). At the end of the day, the points are transfered from the machine and merged with the Excel file. A better way is to use a data recorder that allows additional information to be associated with each point as it is recorded. In this way, when the data are transfered at the end of the day, the complete database is there ready to be plotted.

The question then becomes which data collector to use and how will communicate with the total station. As technology has changed we have tried a variety of solutions over the years. Currently we use three different solutions: one, a netbook running our EDMWin software, two, an HP iPAQ running our EDM-Mobile software, and three, a Trimble Nomad (or Recon) running EDM-Mobile. There are advantages and disadvantages to each. The netbook and iPAQ solutions are fairly inexpensive, especially in comparison to data collectors sold by the total station makers and in comparison to the Trimble Nomad. Netbooks have the advantage of using the standard Windows operating system meaning that other applications can work in conjunction with the data collection software. The netbooks tend to be fast, they hold a lot of data, have a full keyboard, and they can be used for other purposes (like lab work). The main disadvantages to the netbook are that it is a little less portable and the battery has to be managed carefully to make them last an entire day. The main advantages to the iPAQ solution are its portability and battery life. The computer is easily held in the hand and the normal battery last longer than the netbook battery. The main disadvantages are that the operating system is not Windows (and like XP is no longer being developed by Microsoft), the screen is not full sized, there is no keyboard (or a greatly reduced keyboard), and the data are stored in a file format that is not immediately compatible with Microsoft Access (ie. requires a special driver or our software). Because netbooks and iPAQs are fairly inexpensive, multiple machines can be purchased and this is important because in our opinion you should always have a backup machine. Finally, from a hardware point of view, the Trimble Nomad (or Recon) is an excellent machine that essentially consists of an HP iPAQ embedded in a super rugged case with several important built-in features (some optional) including an extra battery (which lasts for at least two days of heavy use), a numeric keypad, a serial port, a barcode scanner, a camera, and a GPS. The disadvantages include cost (the Recon is less than the Nomad but a cheaper solution can still be built up from an HP iPAQ) and potential software incompatibility (it uses the same discontinued Windows Mobile 6.5 operating system that the iPAQ uses).

As for communication, the solution depends somewhat on the choice of data collector, but the three main options are serial, USB, and BlueTooth. At one time, serial communication was the only option and it is still supported by the total stations that we know. The problem is that most computers long ago stopped including serial ports. One solution is to purchase a compact flash serial port adapter (such as this one or this one). The iPAQ computer and many netbooks come with a CF slot and so this solution works fairly well. The main problem is that the cable connection is fragile and easily breaks. So we strongly recommend purchasing more than one serial adapter. Another possible solution is a serial to USB converter. These devices can be less expensive, but our experience is that they do not always work. Note too that they will likely not work on the iPAQ (these devices require drivers not supported by Windows Mobile). For serial communications by far the easiest and most reliable solution is the Trimble Nomad or Recon. These machines have a standard 9-pin serial port built-in.

New total stations also come with a USB communications cable. The ones we are most familiar with (Leica) require a Windows driver which then makes the USB look like a serial device which in turn means that it will be compatible with data collection software that expects a serial port (like our EDMWin program). This is easily the most reliable solution for data communication with a total station. Note that this solution will not work with an iPAQ or a Nomad despite the presence of a micro-USB port on both these machines. This is because the drivers are not available (we checked this with Leica). Lastly, BlueTooth can be used to communicate with the station. We have less experience with this, but we have recently started experimenting with it and the latest version of the EDM-Mobile software supports BlueTooth communication. Both the Nomad and the iPAQ come with BlueTooth support. New total stations also come with an option for BlueTooth communication. For older stations with no built-in BlueTooth, one solution is to purchase a serial to BlueTooth adapter (like this one). When looking for such an adapter, you need to make sure it is battery powered and that the battery is rated for a full day of use. You also need to be sure that the pin configuration is correct (it should be male as the total station cable will be female) or purchase an additional gender-changer. BlueTooth has the advantage of eliminating the cable connection, avoiding expensive/fragile serial port converters on the iPAQ, and allows the data collector to be positioned far from the station itself. As for disadvantages, it can be difficult to install initially though once installed our experience is that it works well. BlueTooth will also use a bit more battery on the data collector and on the station itself if it is built-in (though we do not yet have enough experience to know if this makes a big difference) and if it is not built-in then it is one more device that needs to be charged each night.

One other point to consider carefully when putting together a total station budget is the expense of support equipment. In addition to the total station and data collector, you need to budget extra batteries, extra chargers, extra prisms, extra cables, and a tripod. Our philosophy is that any piece of equipment that is critical to the use of the station (short of the station itself perhaps) has to have a backup. This especially applies to cables. We have also found that, unfortunately, off brand cables and batteries do not work nearly as well and are probably not worth the savings. Prisms can be very expensive and may be one reason to rely on reflectorless recording. For piece proveniencing artifacts it is easy to make a reflectorless target that can be positioned just over the artifact. If you use prisms, depending on the excavation, you may want between two and three prisms per station. And if you plan on doing topographic work, then you will likely need a larger prism and flexible height stadia rod. A 12v to 110v (or 12v to 220v) inverter is also a useful piece of equipment in many situations that will let you use a car to re-charge in an emergency a data collector or total station battery (we have also purchased car batteries to have on site to run equipment for a week at a time - see our paper on the use of 12v). Some total station makers also provide 12v chargers for their batteries, and the same is true for some data collectors. It is always better to purchase 12v chargers for your equipment than it is to convert 12v into 110v (or 220v) and then back into 12v or some other low voltage to charge a device.

If you have any other questions or suggestions about which equipment to buy, you can write us and we will try to respond if we are able to.