Lab RTK GPS User Guide
Projection System / Datum
All GPS and other horizontal position data should be recorded in UTM (Zone 11/12) projection and NAD 83 horizontal datum. Vertical position should be recorded as ellipsoid height (CGVD2013).
A GPS Receiver is a device that provides its geographic position (Latitude, Longitude, and Altitude) information with the help of satellite navigation systems. Depending on the sensitivity of the receiver device and a few other factors, precision of the location data provided by a GPS receiver might vary from sub centimeter to several meters.
An RTK (Real time kinematic) System is designed to collect précised (sub centimetre) position (Latitude, Longitude, and Altitude) data. It consists of two components; a base and a rover. In general, the base is set at a known position, and the rover is used to measure the Latitude, Longitude, and Altitude of nearby unknown positions. The base is connected with the rover via Bluetooth/wireless. The way the system works is that the base communicates with the satellites and estimates corrections for its position and then send it to the rover. The rover also communicates with the satellites and calculates its position and then correct it with the correction data received from the base (Figure 1).
Figure 1: Simple diagram of a RTK system (Source: https://www.instructables.com/id/Sub-Centimeter-GPS-With-RTKLIB/)
However, while working in the wilderness, it might not be possible to find a known position where the base can be set. In that case, base can be set at an unknown location and later post-processing (Precise Point Positioning PPP) can be performed to estimate and apply corrections. To be able to get précised correction estimates using PPP, base should be left at a given location running for at least two hours. There are a number of freely available and easy to use software for PPK (example: https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php?locale=en).
It is to be noted that the base and rover must be within a certain distance (usually 200-300 m) to ensure good communication between them. In addition, this communication works best while they are within the visible line of sight. However, while surveying in deep forest or in mountainous regions, it is extremely difficult to meet either of these conditions. A Repeater, an amplification system that contains active and/or passive antennas, might be very useful in such case. It simply amplifies the base signal so that receiver can communicate with the base from far (1-2 Km).
The BWRAP generally intent to pursue Precise Point Positioning (PPP) Surveys as it might be impossible to find a known position for the base in the wilderness. In PPP survey raw data is collected in the field and local (corrected) coordinates are determined through post processing. However, where possible (e.g., in company leases), we will use existing-control surveys, which deliver ground coordinates in real-time. However, these require tie-in to Alberta Survey Control Markers (ACSMs), which may not be found within range of a given field site. Under conditions where ASCMs are not available, we will move to PPP surveys. In all cases, our goal is to transform GPS coordinates into local coordinates that are accurate in the ~1-10cm range.
The main difference between a mapping-grade survey – one with absolute accuracies in the ~1-10m range – and a survey-grade survey – those with accuracies in the ~1-10cm range – is the correction for local atmospheric and multipath errors. Since these factors vary spatially, they are best measured by a base station sitting in a stationary position in an existing-control survey, the base station is set up on or near an existing control point with accurately known local coordinates. In this manner, GPS coordinates can be translated into local coordinates in real time (Figure 2).
Figure 2. Typical RTK survey setup (Source: http://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/earthsciences/pdf/Canada-RTK-UserGuide-v1_1-EN.pdf)
Accurate existing-control surveys require the operator to set the proper reference system, assemble and configure the GNSS system correctly, tie into one or more high-quality survey monuments, and establish reliable communication between the base station and rover(s). Some of the key considerations include:
- The base station should be set up in a stable position with an unobstructed view of the sky and good radio-line-of-site access to the working rover(s);
- Base stations should be tied into existing control points whose location is known (e.g., Alberta Survey Control Markers – ASCM);
- Communication between the base station and rover(s) should be strong; and
- Only FIXED observations should be recorded. FLOAT and DGPS solutions are normally caused by attenuation by thick brush and vegetation and will seriously degrade the quality of the survey.
It is also good practice to record raw observations in the base station and post-process these data through PPP-survey techniques. This provides both a verification method and a backup for the existing-control survey.
Downloading ACSM Index Cards
There is a network of nearly 30,000 ASCM monuments throughout Alberta, each identified by a brass tablet and often accompanied by a nearby peg or pole (Figure 3). ASCM ID cards contain information in individual ASCMs and are available for free through the Government of Alberta Spatial Information System (SPIN) web site. ID cards should be downloaded prior to surveying and used to locate ACSMs in the field.
Figure 3: Typical ASCM monument (left) and marker (right) (Source: http://www.waymarking.com/waymarks/WME9G_ASCM_91777_Medicine_Hat_Alberta)
When working in a remote area far-removed from ASCM control points, Precise Point Positioning (PPP) surveys provide an alternative means of translating GPS coordinates to local coordinates, through the use of precise satellite orbit ephemerides. The process involves storing raw observations on the base station and submitting them to the CSRS-PPP tool after the fact (Figure 4).
Figure 4: Typical PPP survey setup (Source: http://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/earthsciences/pdf/Canada-RTK-UserGuide-v1_1-EN.pdf)
Accuracies of PPP surveys are subject to many of the same factors affecting existing-control surveys. However, other issues are also at play: the most important being the length of time over which raw observations are recorded. 1-2 hours of uninterrupted GNSS observations can achieve <10cm accuracy and is deemed suitable for this protocol. Greater accuracies (<5cm) can be achieved with 2-4 hours of GNNS observations. 12-24 hours of observations are required to achieve the cm-level accuracy.
An existing-control survey involves operating the base station on or near an existing ACSM control point. While you need not be situated directly on top of a marker to work in this manner, your survey must be tied in so that GPS coordinates can be translated to local coordinates in real time. Note that even though we expect to record accurate local coordinates in an existing-control survey, it is also good practice to have an SD card in place in the base station so that raw data can also be collected as backup.
- Set up the base station in a secure location with good visibility of the sky and an unobstructed view of the area to be surveyed. The tripod should be level and set at its maximum height. Turn on the base station as soon as possible and start the process of raw data collection. If you are fortunate enough to have an ACSM monument in a position that allows you to set up on top of it while maintaining security, good visibility of the sky, and an unobstructed view of the area to be surveyed, then please do so. Otherwise we will tie into the ACSM network through localization in Step 3.
- On the field sheet, note the GNSS base serial number, survey start time, and antennae height.
- Configure the base station and rover according to the instructions provided by the manufacturer. Make sure that the system is recording FIXED observations. If you are not recording FIXED observations (Autonomous and Float observations are the other alternatives) then something is wrong with the configuration, sky visibility, or base-rover communication. Correct this before moving on. Also check the latency, which indicates the frequency of radio updates in seconds. This should be at or near 1.0.
- There are two ways to occupy existing control: (i) with the base station on top of a control marker, or (ii) with the base station nearby an existing marker. Note that the control marker need not be an actual ACSM monument; it can also be one placed previously by other personnel/company and whose position is accurately known. If you are set up on top of an existing marker, then you must accurately measure the height of the antenna above the marker (Equipment: GPS Base: RTK) and configure the base From Known Position. Normally, this will be entered using the Use Local Coordinates interface. If it is not convenient to set up on top of the control marker, then configure the base From New Position: Read from GPS and then enter the control point through Localization. Enter the coordinates of your known points using the File: Points menu. Then, you can add the control point number in the Equip: Localization: Points tab. Make sure that the On/Off function is set to record both the vertical and horizontal positions of your control point.
- Perform the survey (Survey: Store Points).
- On the field sheet, record the survey end time.