OK, I have several dumb questions regarding the Ground Systems
option in Ashtech Solutions V2.4:

1- In the Angle option, I have to admit I do not quite
understand what Grid is referring to when I need to tell
the software what the orientation of my local ground system
is. Does this mean I need to select a grid (State Plane, for
example) and do an adjustment BEFORE I switch to my ground
system - or what??

2- After selecting Angle radio button, the text box for
entering the angle is very weird. I have yet to figure how to
enter a minus sign. And this box does not behave in any
way/shape/form like a standard Windows textbox, but attempts
to control my keystrokes.

3- After entering the base point coordinates - 50000,50000,
for example - after re-adjusting, it is very common to see
the reported coordinates as something like 50000.002,49999.998.
Why come? Not only that, but I have occasionally seen the
Origin coordinates actually change in the Ground System Definition
text boxes after an adjustment. This is almost certainly not
a good thing.

4- Why is the only datum available for orthometric heights
EGM96?? This seems passing odd to me...

I would happy to have someone show me the error of my ways.

- Steve

Steve,

On Ashtech's Web site, there's a Technical White Paper titled:
"Grid to Ground Transformations using Locus Processor/Ashtech Solutions". It's a downloadable PDF.

This may provide you with some answers.

Point your browser to: www.ashtech.com
Then browse to Surveying | Surveying Systems | Tech Talk...
You'll find the Grid to Ground paper at this location.

Bob LeMoine
Thales Navigation
Ashtech Precisin Products

Thanks for the feedback.

This .PDF file appears to be referring to how to define a local grid system and then manually converting to ground values. Also was written for V1.2.

I am after apparent problems in V2.4 in the Ground Coordinate System option, wherein all of these matters are supposed to be handled automatically.

Typically all my projects are done on "real" state plane coordinates, where NGS control is used. I am using Geoid 99 and orthometric heights, so I can't see where you are "forced" to use EGM 96.
Try this. Process and adjust in either state plane or geodetic first, get the project resolved with a free adjustment and then a constrained adjustment.
Then create the local grid. Locus V1.2 took some doing to "estimate" the parameters, but Solutions 2.4 does it automatically. Select your origin site, input the assumed ground coordinates, and if you used a state plane grid, if you set the radio button to "angle" and input 00-00-00 (DMS) what I have found is that it will apply the convergence angle at that point to the project. Locus 1.2 we had to go and look at the convergence angles, change the sign because it was a bug, and apply it manually to the custom grid.
I agree the "angle" dialog box is somewhat funky, but if you highlight the degrees with a right click and a drag, you can change them. I also think if you just highlight the whole angle and delete, you can type it in and it gets it right. Another thing to fool around with to understand its quirks.
Give it a try, do a solar observation between your origin point and one you can see in the field, and check it against the custom grid.
And as far as the - sign in the angle dialog box, say you needed -00 08 32 as your angle, its the same thing as 359 51 28.
I think we discussed this a while back under the thread "Solutions 2.4 tip #1" or such.
Best of Luck.
Dave
Modified By Dave Huff on 8/21/2001 at 11:31 PM

EGM '96 will be the only choice for orthometric heights in Solutions if the WGS '84 datum is being used. If the datum is NAD '83, Geoid '99 can and should be used Vs EGM '96 for orthometric heights in the USA.


BR, Richard

OK, I am apparently not making myself clear, here.

The V2.4 .PDF documentation says the following:

"A Ground system builds upon a Geodetic system only. It has no
relationship to a selected Grid or Local Grid system."

If this is so, then why do options exist in the Ground System Definition Dialog Box relating the Orientation parameters to something
called "Grid"?

To further quote from the .PDF documentation"

"The To Grid North option defines 0 azimuth of the Ground System to
match Grid North."

"The To Angle option defines 0 azimuth of the Ground System as offset
from Grid North by the entered angle. A positive angle results in the 0 azimuth of the Ground System being rotated counter-clockwise from Grid North."

I do not understand what is going on here. Simple semantics, I believe. What the heck is "Grid" in this context??

Regardless, I cannot make heads nor tails of what this rotation "should" be in order to get the "right" coordinates. I am fooling around with a project we did wherein we rotated/translated/scaled Ashtech coordinates using in-house software. I can get elevations and ground distances to agree well with the "expected" results - but NOT the directions.

Rotation and translation are fundamentally simple problems and this part of the software seems unjustifiably obfuscated. If I were writing the software, I would have the (simple) option of selecting 2 GPS points and then simply stating what the Ground Direction should be.
The Locus software had major problems with Local Grid Systems and I wonder if these problems have been incorporated in Ashtech Solutions. I understand that the geodetic folks are not real happy with translating geodetic stuff to a "plane", but it seems that over small site areas - maybe 5 or 10 square miles - the conversion shouldn't be all that hard to do. To answer an anticipated question from readers of this: we did a lot of cross-checking on this particular project between GPS coordinates modified by our own software and traditional total station measurements and got outstanding results, making me a believer both in the accuracy of Locus data and in the process of converting this data to the "real world".

Here is the problem I have stumbled across with the Ground System Dialog Box: if you change the Geodetic Datum in the Ground System Definition Dialog Box, then click "OK", the available Height Systems in the Orthometric Elevations option does NOT reflect the options available in your new system UNTIL you click "OK" and then run the Settings option AGAIN, wherein other options may appear. This was what was causing me to see only the EGM96 datum for orthometric heights.
My confusion here came from changing from WGS83 to CONUS83, and still not seeing any other option than EGM96.

I don't know who writes software like this, but it sure has never been beta tested (at least not by 'dirty' surveyors like me!).

I am willing to bet that the VAST majority of GPS users want to be able to manipulate GPS data in the "real" (i.e., "plane") world. Whoever amongst the GPS community makes this process as painless as possible is going to make a LOT of money! The difficulty in moving data into "real-world" is, with people I have talked with, the Number One complaint. Number Two is, of course, having the GPS operator looking like some sort of Rube Goldberg creation, with wires/cables/batteries/noogies hanging from every possible appendage.

Hello:

When you define a new Ground System, choose the Origin Point with its Origin Coordinates, and select "Grid North", "Grid North" in this case is referring to a Stereographic Projection that is centered at the chosen origin point. The manual is correct in saying that the new Ground system does not relate to any other Grid system that you chose beforehand. The Angle button would refer to Grid North in this Stereographic Projection. This method was chosen becuase these surveys are for a small local area, without the requirement of being tied to the State Plane coordinate system.

As for the Angle sign, if the Angle is Positive, the Ground system azimuths will be larger than the Grid System azimuths. If the Angle sign is negative, the Ground system azimuths will be smaller than the Grid system azimuths. You should be able to choose the correct Angle for your data based on this relationship.

If you choose Geoid '99 and the North American 1983-CONUS datum before setting up your NEW Ground system, Geoid '99 will be available when you set up your Ground system. You don't have to go back and select Geoid '99 again. WGS '83 is not a datum for GPS work.


If youa interested in a cable-less GPS system, our LOCUS system requires no cables in the field. It was designed at the request of our surveying customers.

Let me know how it goes.

Best regards,

Richard

Thales tech. support.

Actually, I just set up a test data set with a Stereographic projection in Solutions 2.40, and compared the plotted azimuths Vs a Ground System with a 90 degree angle.


If you enter a positive Angle when setting up your Ground coordinate system, the azimuths in the Ground Coordinate System will be SMALLER than the azimuths in the underlying Stereographic Grid coordinate system.

Best regards,

Richard

Steve,

You did say you were a Locus user didn't you?

Yes, we have been using Locus for several years.

We have used the Locus software, Ashtech Solutions V1.0, and now Solutions 2.4.

Here are some more responses to earlier questions about setting up a NW Grid System in Solutions 2.40:

(1) The data should be processed and adjusted in State Plane or Geographic coordinates before setting up the Ground System. Use all the adjustment and processing checks that are normally done : Vector QA, network adjustment statics, antenna height blunder detection, etc.

(2)Agreed, there is no way to enter a Negative angle. You could try using (360+ Angle) where Angle is the negative angle that was required. Example : if the angle was intended to be -45 degrees, use Angle = +315 degrees. I haven't been able to try all the combinations on this yet, but in principle it should work. I also cheked this with a Solutions program engineer.

Depending on the what quadrant the Ground North axis is in, and what Grid Quadrant the azimuth between the survey points lies in, a positive Angle can give Ground azimuths that are EITHER larger or smaller than the corresponding Grid azimuths.

(3) It is not necessary to adjust the data twice -- one adjustment in State Plane or Geographic coordinates before setting up the Ground system is sufficient. The purpose is to get reliable vectors to base the calculated Ground distances on.


BR, Richard

I don't really understand your reply.

In particular, I am mystified by the following:

"Depending on the what quadrant the Ground North axis is in, and what Grid Quadrant the azimuth between the survey points lies in, a positive Angle can give Ground azimuths that are EITHER larger or smaller than the corresponding Grid azimuths."

Is this a guessing game, or are there rules to interpreting when rotation will be clockwise and when it will be counter-clockwise? And what the heck soes "quadrant" mean in the context of azimuths?

The on-line help states:

"To Angle option defines 0 azimuth of the Ground System as offset from Grid North by the entered angle. A positive angle results in the 0 azimuth of the Ground System being rotated counter-clockwise from Grid North."

I would interpret this to mean that, if I enter a positive angle, then the effect is that the Ground azimuths would be rotated clockwise, since the 0 azimuth of the ground is being rotated counter-clockwise.

Yet your answer implies that this ain't always true.

=============================

OK, here is some actual processed data from our test project:

State Plane 1983 AZ Central:

Delta E Delta N

0022 - 0037 09/22/99 11:11:20 N/A -48.064 0.011 11760.908 0.023 187.081 0.035 11762.494 0.043 01:14:10 N/A N/A L1 GPS
0022 - 0037 10/11/99 08:15:20 N/A -48.065 0.008 11760.874 0.011 187.154 0.018 11762.461 0.022 04:54:20 N/A N/A L1 GPS
0022 - 0037 10/01/99 07:19:30 N/A -48.082 0.008 11760.885 0.012 187.123 0.022 11762.472 0.026 06:16:10 N/A N/A L1 GPS

Average 22->37: -48.070 11760.889

Ashtech AZ from 22->37: 359 45 56.9

I want to end up with: 359 40 34.9

Thus, I want to rotate my azimuths CCW, which means I want 0-azimuth
to rotate CW, which means I want a negative rotation for the To Angle
option for the Orientation.

So, the rotation should be: -000 05 22.0
or: 359 54 38.0

I want these base coordinates for point 22: 50000.000 N
50000.000 E

For Point 37, I should then get:

Delta N: 11760.799
Delta E: -66.433

After changing these things in Settings, I get the following:

0022 Processed 49999.998 1.#QO 49999.998 1.#QO 2126.456 0.000 Hor/Ver
0037 Processed 49967.564 0.006 61760.959 0.010 2312.053 0.015

This is:
Delta N: 11760.961
Delta E: -32.434

This azimuth is: 359 50 31.2

From my point of view, the rotation has thus been: (CW) 000 04 34.3,
which means that Ground 0-azimuth has been rotated CCW, not CW.

- First, Ground 0-azimuth has been rotated CCW, not CW.
- Second, not only is the sign of the rotation wrong,
but the amount is incorrect.
- Third, notice that the base coordinates are NOT
50000,50000. The difference is small - but disturbing
nonetheless.
- Finally, notice that the horizontal distance between
these two points should be 11760.987, but is adjusted
as 11761.006. Again, small but odd.

The Ground Axis is rotated Clockwise from the Grid axis if a positive Angle is used. Here is an example of what I'm saying about the Grid and Ground azimuths. I tried this with a sample data set in Solutions.


CASE I

Consider 2 survey points PtA and PtB in the existing Ground System that you are trying to duplicate in Solutions

The Grid azimuth PtA to PtB = +30 deg 00 min 00 sec.


If an Angle of +30 degrees was used in Solutions,

The Ground azimuth would be 0 degrees

CASEII

As before, the Grid azimuth PtA to PtB = +30 deg 00 min 00 sec.

Then, say that an Angle of +10 degrees is used.

The Ground azimuth now becomes +20 degrees.


CASE III

As before, the Grid azimuth PtA to PtB = +30 deg 00 min 00 sec.

If an Angle of +60 degrees 00 min 00 sec was used, the Ground azimuth would be +330 degrees 00 min 00 sec

In all 3 cases, the Ground North axis is being rotated in the same direction (clockwise) relative to Grid North, but the resulting Ground azimuths can be larger or smaller than the Grid azimuths, depending on the size of the Angle i.e. the oreientation of the Ground North Axis vs the location of the 2 survey points in the Grid system.

Are the same elevations being used in Solutions 2.40 and the other software package that produced the Ground distance you speak of (11760.987). This could account for the difference of 0.014 (feet ? Meters?)