Local System - Underlying Coordinate System

[Brett Chappell PLS]

Hello Folks,
I see that unknown coordinates has been removed from the software. I actually liked this definition
for a system as it differentiated itself from the geodetic and state plane systems. I see that "Local System" has replaced the "unknown" name. I do like the Local System name. I need some understanding with the new approach to the Local Coordinate System.

When I import coordinates in a 5000,5000,orthometric height system I get a bit lost.

My default coordinate system for the project is Zone is NAD83(2011)/California Zone 2 | NAVD 88.
I import to Page 3 which shows said default coordinate system.
I go to change the Page 3 coordinate system in the Edit Page.
I press the Local System Icon and name the system "Local Control".
I see that the Underlying Coordinate System is defaulted to WGS84(ITRF2008).

This is where my brain train runs off track.
1. My Ref. Frame is NAD83(2011)
2. My default coordinate system for the project is Zone is NAD83(2011)/California Zone 2 | NAVD 88.

Questions:
1. Why is WGS84(ITRF2008) showing since I have not prompted this to my knowledge for this project?
2. What is a underlying Coordinate System?
3. Should the underlying Coordinate System default to my ref frame or Zone?

My thought process is the Underlying Coordinate System is the one used to collect control.

I press to change the Underlying Coordinate System. It takes me to the Coordinates Systems. I go to
add "Unknown" as that was available the last time I used the LS. It is now gone.

Question.
What coordinate system do I use to import my Local System of 5000,5000,orthometric height?

Thank you,

Brett Chappell, PLS

 

[Shawn Billings]

Questions:
1. Why is WGS84(ITRF2008) showing since I have not prompted this to my knowledge for this project?
It's a default behavior of the software. It would probably be better to default to the Project Coordinate System.

2. What is a underlying Coordinate System?
The underlying system is the framework for how the localization will be created. There are several cases to consider with an underlying system - single point/multiple points and 2D/3D.

Single Point 2D:
You arrive on site and want to set up a local system for you project. You want to call your base position 5000,5000 and you want the coordinates to reflect ground measurements, and you want the orientation to be geodetic. In this case, you would use a Geographic system like NAD83 or WGS84(ITRF2008) for surveyors in North America, I would strongly urge users to always use NAD83, unless they have a very good reason for using ITRF2008. The software will automatically populate the scale with the elevation factor for the single point (always good to verify though by looking at "customize" in localize). In this case, you don't care about elevations so selecting the proper vertical datum isn't critical. The local system inherits the characteristics of the underlying system and since the underlying system is NAD83 with no vertical datum selected, such as NAVD88 with Geoid12B, then the elevations will be ellipsoid.

Single Point 3D
Similar to the first case, but you want to have orthometric ('sea level' heights). In this case, you create a coordinate system for NAD83 with NAVD88, Geoid12B and then use this system as your underlying system. Now the reference plane for your elevations is the Geoid, because your new local system inherits the characteristics of the underlying system, and your bearings will be related to Geodetic North at the base. We create a projection centered on that single point, so as you travel east or west of the base, convergence of meridians will apply and you will no longer be "geodetic".

In the case of the Single point localization, if you wanted (like Nate) to use an existing Grid North for the rotation, then you could selected State Plane as the underlying system and you will have State Plane Grid North for your bearings.

Multiple Point 2D
This would likely be for boundary surveyors wanting to find the best parameters for matching coordinates to an existing survey. In this case it doesn't really matter too much which underlying system is selected, because the rotation will be determined from observation. The only difference is that the rotation may be 1° from Geodetic North or 2° from Grid North, if the mapping angle (difference in Grid North and Geodetic North) was 1°. It's only a difference of where you start your rotation from. The same for scale factor. If you select a Geographic system, then the scale factor will be the elevation factor, if you select a grid system, the scale factor will be the combined factor. Again, it will return the same output, it just begins at a different place geodetically. With multiple points, the rotation and the scale are based on your observations compared to the known values. It's very important to know what values to expect (even roughly) so that you can properly evaluate the results the software is giving.

Multiple Point 3D
Like the Single Point 3D, if you want orthometric heights, you need to pick an underlying system with NAVD88 and a Geoid model, otherwise your heights will be relative to the ellipsoid. The rotation and scale will be determined by your observations compared to the known target values.


3. Should the underlying Coordinate System default to my ref frame or Zone?
As I answered above, setting to WGS84 is the default behavior of the software. It would probably be better to default to the Project Coordinate System.

 

[Shawn Billings]

My thought process is the Underlying Coordinate System is the one used to collect control.
This is incorrect. Remember that all surveyed coordinates are stored internally (transparent to the user) as WGS84(ITRF2008) latitude and longitude. Any time you view your coordinates, they are being transformed and projected into the coordinate system you have selected. The Underlying Coordinate System determines the projection used for the localization. The localization then becomes a series of transformation parameters to match that projection to the known target coordinate values. If a Geographic Coordinate System (lat/long) is used, then we create a projection, project those surveyed coordinates onto that projection and then determine transformation parameters between the surveyed values and the known target values. If a grid system is used (such as State Plane) we skip straight to determining transformation parameters to get from State Plane to the known target values.

 

[Shawn Billings]

I press to change the Underlying Coordinate System. It takes me to the Coordinates Systems. I go to
add "Unknown" as that was available the last time I used the LS. It is now gone.
In reality "Unknown" isn't gone, it's been upgraded to have a name: Project Name + Date + Number but it's no different really. Click the icon with a house and plus to create a new local coordinate system (which is what the unknown system was).

 

[Shawn Billings]

Question.
What coordinate system do I use to import my Local System of 5000,5000,orthometric height? Once you create a new local system (house with a plus icon), you can then import using this coordinate system. This can be done before localizing.

When you create the local system, if you the LS has not GPS position, the origin is set to 0° Lat, 0° Long, if the LS has a GPS position, the origin is set to your current position. After localizing the origin is moved to the centroid of your surveyed localization points.

 

[Matt Johnson]

In reality "Unknown" isn't gone, it's been upgraded to have a name: Project Name + Date + Number but it's no different really. Click the icon with a house and plus to create a new local coordinate system (which is what the unknown system was).

I would add that there is a slight difference. Previously, Unknown coordinate systems did not have a defined relationship/transformation to WGS84. When this was the case, design points imported into a local coordinate system could not be staked until a localization was created and saved. Now when you create a local system, the coordinate system chosen as the underlying coordinate system defines the initial transformation to WGS84. If you choose a state plane coordinate system as the underlying coordinate system then the local system uses the same transformation as the state plane coordinate system. It is a copy of the SPCS until it is modified with a localization. If you import approximate state plane coordinates into this local system, you can stake them to look for monuments.

When a geodetic coordinate system is used as the underlying coordinate system, an oblique stereographic projection is created to define the transformation from the new local grid system to WGS84.

 

[Shawn Billings]

Thanks for adding that Matt. That was the primary motivation for the change.

See this video

 

[Brett Chappell PLS]

Thanks for the information.

 

[Nate The Surveyor]

That's downright awesome!

 

[Brett Chappell PLS]

Hi Shawn,
Thanks for the detailed response. This was my typical work process:

Search ties and setting control:
1) calc search tie point using an approximate ortho elevation taken from Google Earth.
2) upload the calc search tie points to a "SEARCH TIE" page using a unknown system.
2.1) Start Base Station, no connection with Jetpack and LS, Call John Evers - TCP Port show 8010 but in the rover setup shows 8002, switch setup to 8010 and continue with fix.
3) find a couple of points to use in the localization which I collect to a SPCS Page "FOUND CONTROL"
4) localize on a found points to aid in additional searches.
5) check the a localized point and the point would be off a crazy amount of miles.
6) would call Matt Johnson who advised me to download the search tie calcs from the LS to a USB and reload to the LS.
7) would localize again and it would work and I continue with the survey.
8) return to the office and DPOS the project using a CA state plane coordinate system.
9) use the Grid to Ground factor to scale the points in CAD and then move the points to the 5000, 5000 system. I then rotate to a Basis of Bearings from a recorded map
to finalize my control.

Localizing to my control:
1) I start a new job.
2) Upload control as described above.
3) Localize on multiple points control points and continue surveying.

I use this process for a few of reasons:
1) My clients like a small coordinate system for ease of use.
2) I rarely use SPCS as a deliverable.
3) I like to return to the job site to verify the control system I created in cad using a new localization. It gives me peace of mind.
4) I have used this process since 2004 (minus DPOS and tech calls) and it has yet to fail me.

Since I have not seen a underlying coordinate system before I was thrown for a loop. I understand the reasoning's behind each of your described processes.
It provides an amazing amount of flexibility.

I probably would of understood the screen better if we have a underlying coordinate system and a overlying coordinate system.

Thanks for the professional support.

-Brett

 

[Shawn Billings]

Even before, we had an underlying system, it just wasn't called such. The "Surveyed CS" in localization was the underlying system. We just made that point obvious by spelling it out when the Local system is being created.

 

[Matt Johnson]

2.1) Start Base Station, no connection with Jetpack and LS, Call John Evers - TCP Port show 8010 but in the rover setup shows 8002, switch setup to 8010 and continue with fix.

This bug was fixed in the most recent version of J-Field, 2.0.6.375

5) check the a localized point and the point would be off a crazy amount of miles.

This occurs if the local coordinates are not imported into the same coordinate system as the design coordinate system selected in the localization.

Design points are associated with a coordinate system when they are created. The coordinate system they are associated with is that of the current page when they are imported. The coordinate system a design point is associated with can never be changed to different coordinate system but the parameters of the coordinate itself can be modified. Modifying the parameters of the coordinate system with a localization will affect how the points relate to the surveyed points and their actual position on the earth.

 

[Brett Chappell PLS]

Matt,
Thanks for the heads up on the Bug Fix.

Prior to going to the field, I imported my search tie points to a unknown coordinate system and named the page Search Ties. I then collected points on a NAD83 Lat Long system. I localized in the field
and the issue occurred. At no point have I ever imported my 5000,5000 design points to a coordinated system other than Unknown.

What was the issue with this process?
Do I need to collect Lat/Long data first and then import my unknown coordinate system?

 

[Matt Johnson]

At no point have I ever imported my 5000,5000 design points to a coordinated system other than Unknown.

You just need to make sure that the same local coordinate system (previously Unknown coordinate system) that the points were imported into is the one selected on the design side of the localization. Previously Unknown coordinate systems were given names with the date appended to the end of them. If there were multiple Unknown coordinate systems, this naming format was confusing and made it easy to select the wrong unknown coordinate system.

 

[John Thompson]

I worked on a project this week that involved retracing three different surveys. I drew polylines of each of them and put each survey on its own page with its own localization. Worked slick for finding corners. A little setup work beforehand made the field work really efficient.

The localization parameters screen reports north and east origin and ground. I'm not sure why they are reported like that and how I would use them. I think of the localization parameters in terms of translation, rotation, and scale. It would make more sense to me if this screen reported north and east translation instead. What use to me is the origin? Am I missing something here?

 

[Matt Johnson]

The origin point is the point the scale is calculated around so it is critical information too. What is the underlying coordinate system in the screenshot you posted? If it is state plane, then by holding the scale at 1.0 you are holding state plane distances and there will likely be some difference if you were to measure the distances with a total station.

 

[Shawn Billings]

That could be useful in cases like you show here.

 

[Shawn Billings]

The origin point is the point the scale is calculated around so it is critical information too. What is the underlying coordinate system in the screenshot you posted? If it is state plane, then by holding the scale at 1.0 you are holding state plane distances and there will likely be some difference if you were to measure the distances with a total station.

looks like the target coordinates are also State Plane(ish) given the agreement in origins.

 

[John Thompson]

I was using my own LDP, so the scale difference wasn't much. I usually leave scale unchecked to match my chain to the other surveyor's chain, but for some reason in this case the residuals for the points I was interested in were smaller with a scale of 1. I suspect one of the monuments I localized on may have been moved and that threw the scale off. Maybe.

I still think delta N and delta E would be more useful. Those origin numbers are look to me like big numbers (millions of feet) like all of the other coordinates and I can't visualize where they are.

 

[Shawn Billings]

I'd be scratching my head over the 6min rotation. Seeing the translation is small, it looks like the target system was based on SPC. But unless the distances were very short or the original coordinates were based on a long conventional traverse, I'd be curious to see where that much rotation came from.