MicroSurvey’s FieldGenius is a versatile and easy to use software package, that is one of the last remaining open-source surveying solutions. Although easy to use, this does not mean that there are easy to make mistakes that people often make. In this article, we go over some of the most common questions we get, what the issue looks like, and how to quickly solve your problem.
Let me give you a real example of a reported case, and see if you know how to fix the problem. A customer reported that their vertical coordinates were below sea level with a point expected around 20 m, actually being recorded at -5 m. As they were surveying an island off the coast of Belize, this was a problematic issue. This caused a great deal of concern and was obviously not desired. Fortunately, the fix for this issue was relatively simple, and we were able to guide the customer through how to reprocess their data in the field.
Do you know what the issue is? It is arguably the most common source for tech support calls we receive here at the office. What was needed a Geoid file. Once the data was reprocessed with a Geoid file, the values made sense.
In essence a geoid file corrects for sea level. In practice what the file is actually doing is correcting for the gravitational distortion of the earth. In the ellipsoidal model of the Earth, the Earth is assumed to be a spheroid. Obviously, the Earth is not a perfect spheroid. With the use of the Geoid model, you can correct for the real-world gravitational distortions.
To model the entire world, would obviously require an enormous amount of data. Therefore, Geoid models are typically broken into different regions to reduce file size. There are multiple different places you can find Geoid files, but it is important to get the right one for your region. For the US the NGS website is probably your best source, and for anywhere else, the FieldGenius site is likely the best location.
Reprocessing in a New Coordinate System
One of the most common issues we hear about can be solved with a little bit of pre-planning, and one simple function. Before getting into the solution, it is important to understand first how base and rover setups work, and one of the limitations of this kind of setup.
With RTK receivers you can achieve a real-time position that is accurate to within a centimeter relative to the base. This disclaimer is the most important part, and one of the most common issues that new RTK surveyors run into.
When setting a base up, the base is selecting a point within a half meter diameter circle and broadcasting that position to the world. This point is more than likely not the exact point over which the receiver is set. This means that your rover point will be out in the same direction as your base.
There is no need to fear, you can correct this offset with a bit of planning. You can either do this during your project, or after your project. You will either need a known point, or you will need to collect a static point to create your own known point. Using a point with known coordinates, you can adjust your coordinates to correct the offset in your data.
I’ll give another scenario, and see if you can determine what the issue is. I had a customer call in who claimed his points were several thousand kilometers incorrect. He swore up and down that he set his receivers up correctly, and claimed the issue was with the receivers.
I’ll give you a hint, it is almost never the receivers at fault. Modern RTK receivers, are incredibly robust, reliable and difficult to fool. A 1000 km bust is nearly impossible. The true issue at hand is the horizontal mapping system.
Despite swearing up and down that he had correctly set his coordinate system, the cause of the issue was an incorrectly set horizontal coordinate system. In the case of this customer, the coordinate system he had selected a UTM zones on the opposite side of the planet from where he was actually working. As a result, when recording a shot, he saw this large distortion in his data. Fortunately, this is a relatively easy fix even if the entire project is shot in the wrong coordinate system.
In order to map the coordinates of a sphere, which your GNSS receivers supplies to you in the form of latitudes and longitudes and an ellipsoidal height, onto a flattened mapping plane, one requires a transformation. If your coordinates lay to the extremes of the transformation boundaries large distortions can occur.
Depending on where you are located, different horizontal systems may be used. In Canada, we generally see a derivative of one of the Universal Transverse Mercator (UTM) zones. In the States, we most often see surveyors working in a state plane coordinate system. Many states have different derivatives of state planes based on the year, make sure you are using the correct system that is relevant for your area, and your survey. However, even if you record points in the wrong coordinate system, you can simply reprocess your data in a new system.
Reprocessing in a New Coordinate System
This last tip may seem very obvious to some of you; however, it is a very common question we receive, especially from newer RTK users. We often get a question along the lines of, “I have processed my data, and my points are half a meter out in a seemingly random direction at every point”. 99 times out of 100 the issue is down to the status of their GPS when recording their solution.
To understand why this random error occurs, one must understand the meaning behind the status of their solutions. For our purposes, we are going to look at the four most common solution types, Autonomous, DGPS, Float and Fixed. Each solution type offers a different level of accuracy, and understanding that position level can save you time in the field.
An Autonomous solution, is often the fastest solution but as a consequence offers the poorest accuracy level. With an autonomous solution, you can at best hope to achieve a solution accurate to within a meter. To achieve this accuracy level, the receiver calculates a position in real-time from satellite signals.
The next step up in accuracy is the differential GPS (DGPS) solution. With a DGPS solution, the receiver is receiving corrections from geo-stationary satellites, known commonly as SBAS (Spaced Based Augmentation System). Depending on where you are located, the name of the constellation may be different, in North America it is known as WAAS, in Europe EGNOS, Japan (MSAS) and in India (GAGAN). With these additional corrections, the receiver can calculate a corrected position to within half a meter.
The last step before the most accurate solution is a float solution. With a float solution, the receiver is receiving corrections from a base station, but has not yet fully resolved the RTK equation. In theory, if a long enough occupation time is taken, while the receiver the receiver will obtain our next solution type.
A Fixed solution is the most accurate solution type and will give you a position accurate down to the size of a loonie (.3 inches). In order to obtain repeatable and accurate results, you will need a fixed solution. A fixed solution is the most accurate position a receiver can obtain in real-time, and to obtain a more accurate solution, you will need to occupy your position for 10-20 hours and post process the results.
Bench Mark Equipment & Supplies is your team to trust with all your surveying equipment. We have been providing high-quality surveying equipment to land surveyors, engineers, construction, airborne and resource professionals since 2002. This helps establish ourselves as the go-to team in Calgary, Canada, and the USA. Plus, we provide a wide selection of equipment, including global navigation satellite systems, RTK GPS equipment, GNSS receivers, and more. We strive to provide the highest level of customer care and service for everyone. To speak to one of our team today, call us at 403-286-0333 or email us at [email protected]
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