When setting control points on a job site with GPS, Static GPS I’m told is the best method.
So Static GPS requires post-processing correct?
And can you use RTK and Log at the same time and still post-process?
Or is it best to leave it as a single solution and then post-process?
Thanks
You can collect RTK and also post process. Just make sure your rover and base are BOTH logging. Then you just have to load the log files into Emlid Studio along. The program will use the base station data to correct the rover log. The corrected result is basically a list of positions IE a tracklog of where the rover went. Then you load the CSV project file into Emlid Studio and it will adjust the stored points using the tracklog.
One thing I noticed, and just made a thread about in the RS section, Emlid Studio doesn’t recognize coordinates that you enter manually. It seems like the base calculates its position every time. I may be doing something wrong. But make sure you have the right coordinates in Emlid Studio. I just have to enter mine every time I do PPK
Thank you!
Hi @mcwllc702,
You can collect GCPs using both RTK and PPK. As Day mentioned, you can do RTK and collect logs at the same time. For Static processing in the Emlid Studio, you would need logs from both base and rover. You can get a single point with the Emlid Studio as well.
I worked on this yesterday, I’m setting control for total station work… First I did it in RTK and at 300 feet between points I was getting 0.04’ in error with the total station. witch aint bad between 300’ points but needs to be better for total station work. So I read when setting points for control it’s best to use static GPS and post process. and RTK for traditional layout.
@mcwllc702 , without knowing your workflow, are you taking into account local point scaling for your work? Without this, you will definitely have large differences (gets larger over longer distances) when you start doing work with a total station.
Joel
Thank you, I actually did today, and after post-processing my points in Emlid Studio and converting lat and long cords in NGS, NGS gave me a scale factor of 0.99993169
I shot with a scale factor of 1.0000000000 and I was .02 out, then I changed the scale to the NGS scale above and re-shot it. and my error was .003
Is there a scale factor you use? I’m really happy with .003 error
@mcwllc702 , Every possible point in the world has its on scale factor relative to the next. This is made more complicated when you start to deal with elevation. For simplicity, you’d generally adopt one point for the site and get the relevant scale factor for it.
Who knows, you might even get lucky and the scale factor is 1.
The relationship of scale factor is only really needed when you are not observing on a flat surface (such as by GPS). Depending on your location and elevation, you are either above, below or on the WGS84 datum, and as you go closer to the Poles, the Eastings get narrower. This means your scale factor is going to be different each time. When you then have a site which also has large changes in elevation, this also becomes important, because you then look for the Combined Scale Factor.
In any case, if the majority of the work is going to be done using terrestrial methods (total station, etc.), I would focus on your total station work to establish your control network. Then pick one mark which is fairly central to your site (my preference), and resolve your GPS datum on this mark (such as through OPUS for some authority on your real world position, otherwise Studio is fine). If you do send off to OPUS, I believe you will get an appropriate Scale Factor and Combined Scale Factor. When you then put your data into CAD (or something similar) and center on this mark, you can then work out psudo-grid co-ordinates.
From there, you can then do a Localisation in Flow with the real ground co-ordinates from your Total Station work (scale 1) and Flow will then scale the site (ground) to fit (grid/GPS). This then makes your ENZ values match (within tolerance for error) when importing or exporting data between platforms. You will notice Flow will also give you, what it thinks, is the Scale Factor based on this Localisation exercise. It should be pretty close.
Hope this helps.
Joel
Thank you very much! A lot of info to swallow, but I will figure it out! I will re-read it until I understand everything! Your time is very welcome! Thanks again
That backsight I shot earlier was at 365’ with a .003’ error with the scale factor applied to the total station, I will study up on what you wrote!
after adjusting my scale on the total station to the one point I favor in the center of the site.
example (0.9999571). So when I get the scale factor for that GPS point and load that into total station is it safe to assume I’m shooting in control correctly with the TS?
@mcwllc702 , it might sound counter intuitive, but generally ground based observations (like Total Stations) should remain at 1 (so that ground distances are correct). To fit this to GPS that should be scaled (by say 0.99995 in this case).
The inverse of this value (1.00005) then gets your GPS distance to ground distance (mathematically, in this case).
Saying that, it depends on your application of this.
For me, when I do normal Civil work, everything is designed as traditional ground distances, meaning normal XYZ metres. Our State Roads Department however, everything is done on Map Grid/State Plane (design and construction), which means all the points are scaled to each other. It’s an absolute headache when I get data from them to incorporate their designs into our traditional methods, but you just have to know which way to scale the data to suit.
Something to think about?
I would personally favour the Total Station Control (at 1) be scaled down to suit GPS considering there must be a reason for using a Total Station on this project. At least you can deal with millimetres over a few kilometres with certainty, best we can get with GPS is centimetres with uncertainty.
Joel
Thank you, Can you explain the inverse, I am really trying to understand.
I use GPS for dirt work etc…, but I’m also shooting in building corners, and I’m using a Lecia TS07 Total Station for that kind of work. with a scale factor of 1 my backsights are .04 off, when I put in the GPS scale factor into the TS my backsights are on .003 error.
I shot in two building corners today, measured the length and I’m 1/2" off of what the building length is. Contract drawings give me a length of 104’ and I inverse the 2 points on the data collected and I get 104.01’ so I’m a half inch off. so giving the scale of my GPS points are you saying if my GPS scale is 0.99995 I should input the inverse of 1.00005 into my TS? Also, the points I got are given to me in the contract drawings and CAD files and they match up.
Just the TS is new to me and I’m trying to learn how to set it up off of GPS Control. The GPS we have is great, we have VRS and an NTRIP Base 120 meters away from the site.
Re-reading your post again twice over…lol I think you mean it is best to scale down the TS to the GPS in my case, we do not have any NGS control stations anywhere near us.
Found this info…
The Scale Factor can be entered directly or calculated using the grid factor and
elevation for the current position. When using the current position, the program will read
the LAT/LONG from the GPS receiver. The scale factor is then calculated as: (State
Plane Grid Factor - (Elevation/Earth Radius)). A scale factor can be used to make gridto-ground conversions between identical points, to adjust the lesser sea level distances
of GPS to the ground (greater) distances measured by total stations whenever you are
above sea level. The grid to ground scale factor is lower at higher elevations because it
acts as a divisor (the scale factor is always defined as “ground to grid”).
In summary, when configured to GPS, the scale factor (typically less than 1 – i.e.
0.9999544) can be used to go “grid to ground” through division and when configured to
total stations, the scale factor (typically less than 1 – i.e. 0.9999544) can be used to go
“ground to grid” through multiplication. It can also be used for any other scaling purpose
desired, but be advised that any scale factor used for GPS is used as a divisor.
Because there are two potential uses of the scale factor (to adjust GPS measurements
and to adjust total station measurements), 2 distinct scale factors are stored and
recalled for any job, one for when you are configured to a GPS instrument and one for
when you are configured to total stations.
***Please note if you are using GPS equipment and you add in Rover Based localization
points in the Equip tab / Localization / Points tab — the GPS Scale Factor shown in the Points
tab will be Greater than 1 {i.e. – 1.0000456}. When you save the Rover Localization and tap the
“GPS” tab you will see the inverse {1/x} value grayed out to 0.9999544 or something less than
Thank you for all your knowledge on this matter. I think I have a way to lay out with my TS with a scale factor of 1.0000000 until we can get some (LOCAL) control set up. All we have is GPS (GRID) control as of now. I am going to try and Localize my FGA data collector to Grid to Ground and use that Combined Scale Factor, to shift my GPS points to the ground. Hopefully, the GPS and TS agree with each other. The project size is around 100 acres. If I localize in the middle of sight in theory it should work out in the size of the sight.
Yes, just use the center of the site. Some people may average the corners and center of the site. Unless it’s really mountainous and has a large elevation change, you won’t see any differences.
If it’s a smail site like yours, I usually just pick the center of the site. Where I’m at, we’re on the edge of our state’s projection. Ground versus grid differences are negligible.
