If The Earth Were Flat…

By Doug Aaberg • April 15th, 2018
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If The Earth Were Flat………….

Cats would have knocked everything off by now…

 

 

….also, it would be a heck of a lot easier to survey.

I just returned from a very fun and informative Carlson User’s Conference that was held in San Diego. I taught five classes and assisted in a sixth over the course of two days. I sincerely hope that I was able to help and inform the individuals who attended my classes but without a doubt, I learned plenty! This is only my second User’s Conference and I have been overwhelmed both times in learning about the different work methods, challenges, and needs of surveyors, engineers, and contractors around the country and world. Did you know that Hawaii surveys in South Azimuths?

Throughout the conference though, one theme did receive a lot of attention: Grid to Ground conversion. For those who survey in mountainous areas, converting from grid to ground is an everyday process while those in flatter areas near sea level enjoy the fact that it doesn’t make that much difference, or so they assume. With the upcoming datum changes in 2022, surveyors and mappers are even more anxious thinking about how their day to day lives may be affected and whether or not their existing survey methods will be relevant in the future.

I had thought that most surveyors lived in a world of ground measurements. That is to say that they measure distances along the ground and report them “as measured.” Anytime GPS is involved, they would convert those measurements to ground distances so their final plat could be replicated without any advanced calculations. Most historic deeds and plans are represented in ground distances because, of course, that’s how they were measured. This seems logical as it keeps survey information consistent and easily relatable to the general public. However, I have spoken to several surveyors who are required to report their final measurements as both Grid and Ground distances and then denote all relative information on their plats for retracement purposes.

I remember my first experience in dealing with grid and ground measurements on a survey in Cuchara Valley, Colorado. The project we were working on had a drop in elevation of several thousand feet from one end to the other. We were supplied grid coordinates on control points and then traversed in between. Each leg of the traverse had to be adjusted individually, calculated then projected back up to the site elevation. I wish we would have had Carlson Software back then.

Before we all move to flat states at sea level, remember that we now have software that makes this whole process straightforward and much easier to contend with. With appropriate set up of data collectors and office software, we can ensure that survey data is being recorded and presented in a consistent manner.

What is meant by Grid to Ground?

One gentleman, (first name Mark), presented one of the best graphic analogies for understanding grid and ground measurements and the effect of elevations that I have seen. I am paraphrasing, and perhaps oversimplifying it here, but hopefully, others will find this useful.

 

 

In the above diagram Imagine that a hole was drilled towards the center of the earth at station 1 and 2. A line is then dropped down each. Respecting the gravitational pull, the coordinates established for both stations would be the same anywhere up and down that line. The distance between however would differ depending on where the measurements were taken. That difference can be expressed as a ratio and is known as the elevation scale factor.

 

 

 

Since the earth is somewhat of an ellipsoid (wider along the equator) measuring distances and angles along the surface becomes inconsistent. To resolve that issue, a model or grid is created that reasonably represents the surface and allows measurements to be taken and more accurately related to each other by projecting those measurements onto the grid. Because of changes in latitude and geology, a single grid cannot sufficiently represent the earth so multiple grids are used which vary depending on location. Alaska, for example, has 10 published projections.

A grid-scale factor can be computed and the combination of the grid and elevation scale factors is known as the combined scale factor or CSF. Because the grid model is not perfectly aligned with the ground (there is a whole field of study on this), there is distortion depending where you are on the grid. In areas where the grid is closest to the ellipsoid, you will see less distortion than areas where the difference is greater. The combined scale factor will therefore also vary, depending on your location.

For a great video on this, click here: https://www.youtube.com/watch?v=iwCAAsRvNW4

 

What are some problems?

One of the biggest problems I have seen with recently is just simply that the user does not know exactly what measurements are being collected. With the readily available and relatively inexpensive GPS receivers compiled with published networks, some of which are free, it is commonplace for surveyors, engineers, and contractors to own and use GPS for their surveys. It is also a common practice to use a traditional or robotic total station in tandem with GPS for a complete survey. Canopy, buildings and other site conditions often times dictate this practice. It depends on the equipment, but if you are using a GPS receiver, there is a good chance that you are collecting grid measurements while you total station is likely to be collecting ground measurements.

One workflow I see is GPS being used to set a handful of control points making it easy to establish a starting N, E, Z and a state plane bearing. A point is then occupied and as the survey is performed, other GPS points are “checked in to.” During these check shots, a quick inverse will show what is perceived as an error. Knowing that the total station is going to measure more precisely than the GPS receiver, that error is typically accepted and the survey moves forward. The problem is that what the surveyor may be seeing as an error may actually be distortion.

On a small site, this procedure may be of little consequence but on larger sites, this amount of distortion can really add up. Seeing a difference of more than 0.20’ in a 2,000-foot line is not uncommon. If no post-processing is performed, this difference becomes an unknown error in the survey.

Another problem I have seen is not using or using the wrong Geoid model but I’ll save that for another blog.

 

What are the Solutions?

Fortunately, there are in fact solutions and they are not difficult to implement in your everyday workflow

 

In the field:

1.     From the main dialog box, select Equip

2.     Select Localization

 

3.     Select TS for Total Station

You will see an option to set the total station measurements to be converted from ground to grid.

If you select this option you will see a calculator button that allows you to enter a scale factor with separate calculators that allow you to manually enter coordinates with options to average the scale factor.  
4.     Similarly, select the GPS Tab

You will see an option to scale your GPS measurements to ground distances

 

 

If you select this option, you will see an instrument and calculator icon appear.

 

By pressing the instrument icon, the program will read directly from the GPS unit.

 

The calculator option allows you to control the GPS reading and/or the establishment of the combined scale factor.  

 

Obviously, you need to be consistent and decide whether or not to work in either grid or ground.

For another great video on this, click here: https://www.youtube.com/watch?v=22qjTzfyLqE

 

In the office:

1.     From the Drawing Setup dialog box, first set the projection you are working in.

2.     From the Report distance Scale select your desired scale factor method.

·       Fixed sets a single scale factor for the project

·       GRD>GND scales distances from gird to ground based upon the location along the grid

·       GND>GRD scales distances from ground to grid based upon the location along the grid

 

  1. If you choose Fixed, select the Set button to the right

This allows you to calculate the combined scale factor from the project data.

 

Note: as another option, you may localize your project which allows you to define known coordinates or work in an assumed datum while relating to actual grid values.

 

To carry this option further, you can set Carlson to annotate both grid and ground distances on your plat by configuring the Annotation Defaults.
The result is both grid and ground distances being annotated

 

Similarly, the inverse command can be configured to display both distances.  

 

Post Processing

Survey data can be combined and processed with both grid and ground measurements using SurvNET. This least squares program allows the input of traditional traverse measurements along with GPS vector data in different units, to be processed and combined in a single network.

 

 

With a projection set, the program will adjust the ground measurements to be in common with the grid, process the entire network and allow you to export to a separate scaled coordinate file.

 

 

It is also important to mention that several states and countries have adopted Low Distortion Projections (LDP) which basically breaks up the area into smaller grids where the grid and ground distances are much closer in value. Wisconsin for one, I believe has a separate projection for every county.

It is also possible to create your own LDP with either SurvCE/PC or Carlson Survey. But that also will have to be the topic of another blog.

Another topic of still great interest is Carlson’s Field to Finish program. I had two classes, both full, on setting up and using this great program for optimum benefit. If you need help getting started, download my free Easy Start Guide here:

Download Doug's Field-to-Finish Guide

 

Doug

Let’s Grow Together.

Douglas L. Aaberg, PLS

Survey Product Manager

P)617-393-2300×419

daaberg@carlsonsw.com

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