Wheezing the Base Juice

The base station antenna position is NAD83(2011) per the water district's statement (Lat/Long/Ell Ht). At the rover end, you get to pick the projection and coordinate system, whether SPC or the LDP, along with which Geoid you want to use to get orthometric heights.

Is the base station's position published either by NGS or in a Record of Survey? Looking at the NRS 327.060, only if you are basing your coordinates on SPC you are required to provide where the coordinates are based from and the accompanying accuracies need to be stated. The bases station wouldn't be using a geoid since it it is obtaining X,Y,Z coordinates and broadcasting corrections to your position. Your controller would be applying the conversion to state plane or WGS 84 (this raw LAT, LONG may not be NAD83(2011)).

You can look at the coordinates for the base station in your controller and see if they match published? information. You can change how the points you gathered are displayed in the controller as being ground, grid WGS-84 under point manager, options.

The geoid applied using OPUS will be 12A since that is the accepted GEOID model by the NGS. For the ortho, I would suggest holding the best point and leveling through the rest. As with any GPS determined ortho, the accuracies may vary.

But if you are not publishing the coords as state plane does this matter if the data gathered is accurate to itself?

Brad Luken, LS
_{(AZ, CA, NV & OR)}

SSP,

That is EXACTLY what I would like to do, but I'm a little short on particulars. How do you make the Rover's coordinate system different than the Base? I'm using Survey Controller 11.33.

Dave

What coordinate system do you want your rover to be in? The base is broadcasting NAD83 (2011) per the metadata shown in your initial post. If you want SPCS then you set your coordinate system to NAD83 in Survey Controller and choose your zone. The base is broadcasting LLh (actually ECEF XYZ); you should always use Geoid 12A to bring NAD83 (2011) ellipsoid heights to ground.

If you're looking for some other coordinate system than SPCS then you're probably going to need a site calibration. Or you could let the TSC2 compute ground coordinates based on the site location.

Thanks, Brad. Point manager>>options does indeed look like a place I need to investigate:

"The bases station wouldn't be using a geoid since it it is obtaining X,Y,Z coordinates and broadcasting corrections to your position."

I'm confused. I thought that the base and rover had to have the same coordinate systems. I assumed the Water District Base would convert from WGS84 to NAD83 to State Plane, and then transmit that information to my rover. I don't understand the differences involved in using someone else's base.

My topo was for a PE to make a Plot-and-Grading Plan and a Utility Plan for a custom home. For that I need an elevation referenced to something "official".

Dave

Lee,

"What coordinate system do you want your rover to be in?"

I wanted to do the topo with a total station, so Ground works best. The whole point of using GPS was to get some control points with good elevations. Something I could use to fit my Topo to the record map. I think I failed miserably.

Here's where my understanding breaks down. Don't you have to assign a coordinate system when you create a new job in the data collector? And how do you reconcile GPS measurements with total station measurements within the same job?

Dave

Dave -

Generally you do want to assign a coordinate system when you create a new job; the exception to this is if you are going to do a site calibration to a known local system.

As I said, your RTN is transmitting NAD83(2011) reference coordinates. The NAD83 coordinate system in your data collector is a null transformation, it projects the LL to SPCS without doing any translation, rotation, or scaling. Geoid12A will give you good elevations from the NAD83(2011) ellipsoid heights.

Depending on the size of the site and the combined scale factor, you can probably get by with working with State Plane coordinates. As far as how the GPS data and total station data play together, if you hook the TSC2 to the total station Survey Controller takes care of applying the scale factor to your measurements and everything stays in true State Plane.

If you're copying the coordinates derived from the GPS to another collector, AND if the size of the site and the local combined factor warrant it, most software will let you input a scale factor.

It is also possible to create scaled ground coordinates in Survey Controller by going to Page 2 of the Coordinate System dialogue and changing the coordinate type from Grid to Ground; the software will then let you enter the LLh of your site and it will scale the coordinates based on the combined factor at that point. But I don't really recommend trying that unless you are VERY confident in what you're doing.

What you're looking at in that screen snap is the options for how a point is displayed; it has nothing to do with how it is derived.

Unfortunately there are no short, forum-friendly answers to your questions.

Dave,

It looks like your confusion is caused by the term 'coordinate system'. As refresher, a coordinates system is the the result of the coupling of a specific reference system and a projection. The reference system in this case is NAD83[2011 Epoch 2010] as stated by the Water District. The position of any point in the reference system is reported as either an earth centered cartesian coordinate or a lat/long/ellipsoid height. Only by running a positional value from the reference system through the projection do you end up with a grid coordinate. In the case of western Nevada that would be Transverse Mercator (the parameters are specified by NGS).

In the case of any RTK system, the base send out its position based on the reference system. The rover then uses that along with the correction data to determine its own position in said reference system. It then applies the projection you specify to give you the grid value. It has been a while since I have fired up one of my Trimble data collectors but if I recall, you can view either the projected values or the reference system values (which will be the lat/long or cartisian values). The geoid model is applied to the ellipsoidal height to estimate the orthometric height of the coordinate. In the case of NAD83[2011] you would use Geoid12A to estimate the NAVD83 orthometric heights for you coordinates.

The manufactures confuse this whole thing by acting as though WGS84, the reference system GPS is based on, is the same as NAD83, the reference system our current coordinate systems are based one (There are slight differences in their definitions and center of earth mass) and us a zero transformation between the two. That is why when you look at the local it will be the same as the WGS84. Every RTK system I'm aware of broadcasts its values in some flavor of NAD83.

Just my .25 worth. IMU processing done, time to get back to work.

Lee,

"Unfortunately there are no short, forum-friendly answers to your questions."

I'm amazed at the difficulty involved in learning about surveying technology. If I was Trimble, I'd want every surveyor on the planet using my products like a boss. Instead, Trimble seems to make everything almost impossible to learn. I guess their thinking is that after paying a fortune for the gear, you'll pay another fortune for "training".

Dave

John,

"It looks like your confusion is caused by the term 'coordinate system'."

No, John. My confusion is normal; it's a non-curable, pre-existing condition.

Here's what I'd like to do:

I read all about it in Trimble's RTK Training Rev D. What they don't explain is which buttons do I push?

Dave

Just a quick side question: Pauly Shore, is that you? 😉

Wendell,

I'm glad somebody caught the reference!

Dave