An experienced surveyor has probably heard of survey marks and knows exactly what they are. But those who are just starting out may not be familiar with them. Also known as geodetic marks or benchmarks, they are a crucial part of any geodetic project and represent points in the world with known geographical coordinates.
These survey marks are much more valuable than you think and can be used for a variety of surveying methods. Where they have proved to be more than valuable is when collecting RTK and PPK data. To help you bring your surveying arsenal to a whole new level, we want to give you an insight into this, perhaps basic, but very valuable way of collecting data that can actually turn out to be more valuable than you may think.
What are Survey Marks Used for?
Survey marks are points in the world with already familiar geographical coordinates. You will recognize them as metal disks on the ground or concrete monuments, or other places. There is actually a network of survey marks that exists across the world and there are various national agencies, like the American National Geodetic Survey, that keep track of them and their coordinates. Next time you come across one of them, you will notice that it has the credentials of the agency it belongs to engraved on it.
When collecting data with an RTK receiver, you have to set one of them as a base. Getting precise base station coordinates is crucial for the result of the job you are doing, especially when it comes to topographic surveys. Being that survey marks already provide accurate and well-known coordinates, you can definitely use them to get more accurate point collection.
Different Methods of Setting up the RTK Base
Setting up the base is the first step to ensure that proper corrections can be sent to the rover. However, there are a few different methods that can allow you to do it and achieve proper precision. There are actually three main methods that surveyors usually use to help them achieve proper precision.
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Averaged Single: This is a quick and easy method for when high absolute accuracy isn't necessary. The base station averages its position without external corrections, which may result in an offset of several meters from the actual location. However, all rover measurements will be centimeter-precise relative to this base position. This is useful when you only need to know the precise location relative to a reference point, such as laying out a grid on an excavation site.
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Manual: This method is used when you need high absolute accuracy and have access to a trig point (a point with known coordinates). The absolute accuracy depends on how accurately the point's coordinates were determined. You'll need to enter the coordinates of your benchmark by hand and measure the height between the marker and the antenna.
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Average Fix/Float: This approach provides high absolute accuracy by obtaining Ntrip corrections over the internet. This is useful when you don't have a permanent survey mark nearby or the reference station is more than 60 kilometers away. You can install your local base to reduce the baseline and maintain precise measurements. If the baseline between the local base and an Ntrip station is too long, you can use an averaging float solution, which is generally more precise than the averaged single technique.
Once your base is set up, you can continue with configuring the corrections output and can move on to collecting data with the rover.
How to Set Up a Base Using a Survey Mark?
The responsibility of keeping a record of survey marks is assigned to the national agencies. They have a datasheet which incdlues all the survey marks spread out across with world alongside their exact coordinates. However, you should that while most of the data available is free to access and use, some agencies still prefer to keep it limited and can only allow access on special request.
Some of the agencies in question that keep a log of all the survey marks include:
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NGS datasheet (USA)
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Geodetic Reference Systems Tools (Canada)
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Institut National de l’information géographique et forestière (France)
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Ordnance Survey (UK
After you find the exact coordinates of the survey marks, you can place your base above the mark and then enter the coordinates manually. Be sure to keep the base as accurately as possible over the survey mark, it is important to do so if you want to get precise coordinates. Also, it would be better if you have an open skyview, without any interference. Without those conditions, you are going to need to do a base shift.
You are then going to have to do the following:
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Proper base setup in the field: Lock the adapter into place and add an extension pole. Screw on the receiver and move the tripod over the known point while making sure that you are level and centered above the known point.
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Entering base's coordinates in the app: Power up the base and enter the antenna height and the exact known coordinates manually.
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Make sure to check that you base and the rover have the same coordinate system before you start.
Setting up a Base Shift
If it happens that your known point is inaccessible, you can do a base shift. It this case, all you need is two points, your known point and a measured point from the rover of the known point. Be sure to place your base somewhere near your inaccessible known point and then make sure that you have an open skyview without any interference. Place your tripod over the new point and dial everything. Finish by doing the normal measurement to get the height of the instrument.
After powering your base connect it to your app. In this case, you are going to need to average the base position in singles. Set up RTK on your rover using LoRa radio or corrections from Ntrip. Once your rover is receiving corrections, you can manually add points with the coordinates. Finally, use the rover to collect data on the permanent survey mark.
Use the rover to shoot a point of the known point. You will then get two points, one entered manually and the other measured. To calibrate, select both the measured and the known point in your app and apply the base shift. Once applied, the rover icon on the map will appear shifted.
Conclusion
Survey marks are a fundamental and often underutilized resource for achieving high absolute accuracy in RTK and PPK surveying. By providing pre-established, precise geographical coordinates, these benchmarks enable surveyors to set up their base stations with unparalleled accuracy, which is critical for projects demanding meticulous positional data.
While various base setup methods exist, directly using survey marks through manual coordinate entry offers a robust and reliable approach to minimize errors and enhance the overall quality of survey data. Understanding and effectively employing survey marks, even with techniques like a base shift for inaccessible points, empowers surveyors to elevate the precision and reliability of their data collection, ultimately leading to more accurate and dependable surveying outcomes.
FAQs
What are survey marks and why are they important in surveying?
Survey marks, also known as geodetic marks or benchmarks, are physical points on the Earth with known, precise geographical coordinates. They are crucial because they serve as reliable reference points for surveying projects, enabling surveyors to establish accurate base stations for RTK and PPK data collection. This leads to higher precision in measurements compared to methods that don't use external corrections.
How can I identify a survey mark?
You can typically recognize survey marks as metal disks embedded in the ground, concrete monuments, or other permanent fixtures. They often have engravings indicating the agency responsible for them, such as the National Geodetic Survey (NGS) in the USA.
What's the main benefit of using survey marks for RTK base setup?
The main benefit is achieving high absolute accuracy. Since survey marks have precisely known coordinates, using them to set up your RTK base allows for highly accurate corrections to be sent to your rover, resulting in centimeter-precise measurements relative to a globally recognized coordinate system.
Where can I find the coordinates of survey marks?
National agencies are responsible for keeping records of survey marks and their coordinates. Examples include the NGS datasheet (USA), Geodetic Reference Systems Tools (Canada), Institut National de l’information géographique et forestière (France), and Ordnance Survey (UK). Much of this data is often freely accessible.
