Difference between revisions of "Fieldwork"

Revision as of 23:22, 3 June 2010

Reach site, hunt around,

Setup, precise centering,

Yellow box takes data.

- A concise guide to GPS fieldwork

Perhaps the most important thing to remember about GPS fieldwork is that with a 24-hour GPS survey you can measure the position of the GPS antenna to within about 2 mm horizontal precision and 6 mm vertical precision, and you can measure the vector between two GPS antennas even more precisely than that. What this means is that the tolerances for setup error are very tight and you have to be very careful. If not, YOU are likely to be the limiting error source in the measurements. It also means that you really can't hide it if you screw up. So do your best if you make a mistake or something odd happens, carefully record it on the log sheet. It is far better to know we have a bad observation than to find out years later.

Where can I find site information?

We maintain a GPS Site Database that in theory stores site information from all the GPS sites we use. In practice, some are missing, but the database is mostly complete for Alaska and environs. You can search the database by 4 characeter ID, or radial or rectangular searches, and other ways as well. To access the database, you need a login and password, and for that you need to ask Jeff.

Where to find Julian Day Calendar

Here is printable Julian Day Calendar: Julian Day

Traveling in Alaska

Before you go on official University travel be sure to fill out a Travel Authorization (TA) form. You can get this from Candice Lake on the 6th floor in the buisness office. Even if you do not expect to spend money on the trip, it is important that you fill out this form for insurance purposes. When you return it is necessary to fill out an expense report (to be sure you are reimbursed for incurred costs), even if you had no expenses.

Rental Cars
A rental is often the cheapest way to travel on the road network in Alaska. If you are traveling on University buisness (fieldwork falls in this category) then the university covers the insurance, be sure to decline coverage when renting the car. Be sure to print this document when traveling in a rental car; it contains proof of insurance and some phone numbers to call if an accident occurs. The GI recommends that you not rent from Budget due to their aggressive collection agency should damages occur to the vehicle while in your possesion. The GI recommends using Avis or some other rental company. Do not rent from the airport rental location, there is an additional 10% tax plus fees added to the price of rental if you rent from the airport. Most of the rental companies have other locations in town, which have lower costs.

Flying in the Bush
This is not your regular commercial flight, get all the information you need prior to departure.

How to set up a tripod

There are as many different explanations as people who have set up a tripod before. Well, one description you can use is this: Follow the link to a printable setup description and/or the link to new site and tripod setup description.

How to set up a spike mount

1) Locate benchmark with hand-held GPS or site description.

2) Assemble spike mount by attaching legs to spike.

     a.	*Out of the case the legs on the spike mount will be short.  Use the leg extensions as needed to improve stability.
     b.	Additional wing-nuts, lock washers and spike tips are included in the case

3) Set the spike on the benchmark with the legs oriented at 90 degrees from each other, or as close to 90 as possible.

4) Loosely tighten the wing-nut on the leg extensions to hold the leg lengths.

5) Use the carpenter’s level and the leg screws to level each leg independently.

6) Loosen the wing nuts and add weights (rocks) to hold the leg positions, retighten the wing nuts.

7) Attach the leveling plate to the spike and place the bull’s-eye level on the plate.

8) Use the leg screw to level the plate. Once the plate is level tighten the locking nut on the legs.

9) Remove the leveling plate.

10) Attach the antenna to the spike. ** Do not attach the antenna cable yet.**

     a.	Make sure at least one spike clamp is tight, to ensure that the spike doesn’t rotate when tightening the antenna.

11) Orient the antenna to north.

     a.	Set the declination on your compass, the compass can be placed directly onto the antenna.  
     b.	Some antennas are marked with a North arrow others are not.  If there is no North direction indicated, Notch 1    
        (located at the edge on the antenna) should be pointed north.
     c.	The antenna will likely need to be rotated.  Loosen the spike clamps and rotate in the clockwise (tightening)
        direction.

12) Hook the receiver up to power.

13) Attach the antenna cable to receiver and antenna.

Orienting the antenna to TRUE NORTH

Every antenna has a reference line or arrow that must be oriented toward TRUE NORTH. The reason for this is that the position actually measured by GPS is something called the antenna phase center, which in general is not located on the axis of the antenna. So if you orient the antenna in some other direction, you introduce an error into the resulting postion that for many antennas is much larger than the measurement uncertainty.

The reference arrow for the Trimble 4000's antenna is marked on the antenna.

For other antennas, the antenna cable connector should point to NORTH. There will normally be something marked on the top of the antenna along that same line.

To orient the antenna, you need to know the magnetic declination. It should be recorded on the site description. If not, it can be found on any USGS topo map. If all else fails, remember 25 degrees east, which is not too far off for most of central and southern Alaska.

If the declination is EAST, set your compass to 360 - declination and orient by the north arrow on the compass.

If the declination is WEST, set your compass to 360 + declination and orient by the north arrow on the compass.

If you find that you oriented the antenna the wrong way, and it is already recording data, don't do anything rash! The best thing to do in this case is to carefully record the actual orientation of the antenna.

Programming a Trimble 4000 receiver

You can find a printable description of a multi-day receiver setup by clicking on that link.

Using the Trimble 5700 and R7 receivers

These two receivers are basically the same thing. Neither one is convenient to program in the field. Turn them on and they should work.

What the lights mean.

Using the Trimble Net-RS receiver

The main issue with the Net-RS receiver is that it writes all data for one UTC day into ONE file. This must be considered when a station is moved within one UTC day between sites. It is possible to untangle the data but still it's a hassle. In Alaska a new UTC day starts at 4:00 pm, meaning that taking the station down before 4 pm and setting it out somewhere else after 4 pm would be ideal.

If you must visit 2 sites on the same Julian day, here is a way to get the data to go into separate files. You'll need a laptop, and a crossover (Xover) ethernet cable. Set your network settings on the laptop to IP: 192.168.1.3 NetMask: 255.255.255.0 Gateway: 192.168.1.1 Connect the laptop to the NetRS. Open internet explorer or equiv. and go to the site 192.168.1.2 The web interface should appear with a blue background. Under Data Logging, switch from one 30sec 24hour session to another by toggling the appropriate "enable" boxes. If you need to program a second campaign session, read the manual.

ATTN: The Net-RS receiver has different power and antenna inputs! Make sure you have the right cables (usually they do not fit into the black case) before you leave the vehicle, especially when the site is away from the road.

Using the Topcon GB-1000 receiver

There is printable introduction here: Topcon Guide

Using the Topcon RTK setup

This deserves its own site Topcon RTK

Downloading the Trimble 4000 receiver

1. Get DOS Laptop

2. Boot and cd to C:\DOWNLOAD

3. Connect Serial Cable (Port I/0 1)

4. Connect power (either 12V car battery or adapter)

5. start program '4000': C:\DOWNLOAD>4000

6. rename files according to Julian day

7. follow screen instructions (make sure connection rate is the same as in receiver)

8. get data on memory card

9. ftp data from memory card that should be pushed into a different computer to linux system

Downloading the Trimble 5700 receiver

We will use the "Windows system" computer to download data from Trimber 5700 receiver. And also make sure that log sheet is at your hand.

1. Take the Trimble 5700 receiver out from the yellow box (I know it's "crap", but it's exact what we do first).

2. There are two cables needed to plug into the receiver: power cable (circle); data cable (a little bit rectangular). Power on the receiver.

3. Double click "Shortcut to DataXfer.exe" on the computer desktop. If the receiver is already connected, the software will display the receiver and its series number, check the number and what was recorded on the log sheet, make sure they are matched.

4. Click the "add" button, then you can see all the recorded data in the receiver. Choose the data you want to download, and make sure save the data file to directory E:\GPSdata\. For example, if we want to download compaign data of KENAI trip in 2008, the subdirectory can be created as E:\GPSdata\data 2008\campaign\kenai, and the downloaded data should go to there. Then, click "Transfer", then you will see the computer doing its work to download data. And you need to click "OK" during the download.

5. Change files name. Usually, there are two files for one day of one site: *.dat and *.T00. Change the first 4 letters to site name for both of these two files. For example, for site DAHL, the data file name should be changed to the format like this: "dahl1430.dat" and "dahl1430.T00" (143 is the Julian day). If the name of site has only 3 letters (site CPR, for example), then names should be like this: "cpr_1450.dat" and "cpr_1450.T01".

6. The download work is done. Power off receiver and put the receiver back to the yellow box. Check if all the stuff (receiver, antenna, power cable, data cable) is the box before you close it.

Then you can go on downloading next receiver. You can re-start the software (Shortcut to DataXfer.exe), then it will detect the new receiver automatically.

Downloading the Trimble Net-RS receiver

What goes on the data log sheet?

Data log sheets are used to record important information regarding the site/benchmark and the survey session. We record information about the site, such as the name, benchmark inscription, and location. It is important to log information about the equipment used; including serial numbers, and type of equipment. We record hieght information, direction, as well as time and duration of the survey. It is important to log who conducted the survey as well as any unusual things/mishaps that occured during the survey. Follow the link to a printable log sheet.

Where do data log sheets go

In the filing cabinet, in a hanging file stored by GPS campaign.

Every year, we send a copy of all log sheets to UNAVCO for permanent archiving, along with the data files. Sometimes after a while it is easier to find the information from the UNAVCO Data Archive than it is to find our own log sheets in the filing cabinet.

Converting files to RINEX

The final step in the fieldwork is to convert the data files to the RINEX format. RINEX stands for Receiver INdependent EXchange format. It is an ASCII format that is publicly documented and agreed upon worldwide. In contrast, the Trimble .dat and .t00/.t01 files are proprietary formats that are only used by Trimble. Naturally, every receiver manufacturer has its own proprietary format. There is a more compact binary format called BINEX that is meant to store the same information. BINEX has been adopted as the standard format for PBO, and may one day become the standard instead of RINEX.

There are enough steps in the process that a complete guide is on its own page.

Field Notes

Field Notes on successful and not-so-successful field excursions may help you plan your next trip.