In our previous post, we told you about the Localization feature in the Emlid Flow app, and some of you asked how Base shift differs from it. Let’s take a look:
Apply localization when you don’t have the coordinate system parameters, or they are not accurate enough.
Localization is the process of comparing sets of known and measured point coordinates to calculate or adjust the parameters of the local coordinate system. To do this, the app applies a series of transformations and adjusts all the points within the project. The process requires a set of data—starting with the control points and ending with the available coordinate system parameters. To learn more about the workflow, see our guide about performing localization in Emlid Flow.
Apply base shift when you cannot place your base over a known point because it’s located in a hard-to-reach or obstructed area or when, for some reason, you cannot use it anymore.
Base shift is the process of comparing the known and measured coordinates of one and the same point to shift the rover’s position. To do this, the app applies a basic linear shift to the coordinates of the base in real time, and all points collected after base shift are positioned exactly relative to the adjusted base’s position. All you need are the coordinates of the known point or a permanent survey mark, no additional data is required. See our guide about applying base shift in the Emlid Flow app for the steps.
That’s about it! If you have questions, leave a comment below or contact us at support@emlid.com.
Being able to apply a base shift requires that you are able to use a rover on the Known Point in order to calculate the shift.
Take this example. You have a Known point on the sidewalk at a busy intersection near where you plan to survey. Instead of setting up your base on that Known Point next to traffic, you want to set up your base in a parking lot nearby instead, but you want all the points that you collect to reference that Known Point, not where you are setting up your base in the parking lot. You could use the base shift workflow for that.
That was a very easy good explanation! Basically instead of leaving the BASE sit in the intersection, can just grab the point quickly dodging cars with the ROVER! ; )
Question:
base shift only applies to the rover, it corrects the data there?
Using the example above with the dump truck
I use my base as an RTK NTRIP output after measuring a few GCPs.
When I measure a few benchmarks, they are all off by 35mm North, I need to adjust my base position by 35mm South to get them all spot on. Using base shift would do this easily if I was only measuring and recording GCPs, but I’m also flying RTK.
My base needs to be accurately setup relative to the last time it was used on this site to get accurate cut fill measurement with my RTK drone. Applying base shift in this case would do nothing for that accuracy.
Is there an easy way for me to get the data on how many degrees lat/long I have to add/remove from my base position?
Am I wrong in my assumption that the base coordinates are unaffected by base shift?
Joe
Yes, it affects only the data collected after applying the Base shift to the project.
Hmm, so you enter the coordinates of the base manually, measure the benchmarks with the rover, and see the same shift on all of them? What do you use as a base? Another benchmark?
Technically, it’s possible to recalculate the position of the base, considering this shift. But to achieve consistency in the measurements, you usually need just to place the base over the same point and enter the same coordinates you used before.
A base shift is not required if all you care about is being relative to the local coordinates. A localization doesn’t care what your geographic coordinates are as long as it is an averaged point or a manually entered point that is “close”. We have noticed that if the manually entered point is too far from the receivers initialized location that it will not converge and fix on the rover side. I have not been able to figure out what is close enough but I am guessing within 30m or so from what I have experienced.
A base shift is for after the fact if you do get a known coordinate then you can move all your data along that shift. I does not account for scaling or rotation like a localization does.
Your workflow looks correct. The other suggestion we can offer will mainly depend on the accuracy you need relative or absolute.
If you need points in your survey to be located with centimeter-level accuracy relative to the base and towards one another only - it’s unnecessary to know the precise base coordinates. So you can easily place your base locally and use coordinates from the RINEX header in post-processing.
However, a precise base position is needed to achieve high absolute accuracy. The easiest way here is to set the base over a known point, but you can also place your base and get its coordinates using Average Fix (centimeter-accurate) or PPP service (up to 2 cm accurate for Reach receiver).
If i have to fly a site that has the targets(also used as gcps) set by the client that tie into the sites control, where ive been given those known coordinates of the targets/gcps to tie into as i cant set my base up on the target (as well as the CS for my project)…
would the base shift method work correctly in this case?
Where:
*Prerequisite my project in emlid flow is set to the clients CS
set up base in unknown position
average single and store base point
connect rover via ntrip to base
measure the target/gcp and store as measured point
select measured and known then apply the base shift
disconnect rover (if not using caster service only local ntrip) and connect drone to base via local ntrip for rtk corrections
With the base shift, the base still sends the rover the ECEF coordinates that were determined during the base setup; the calculations are applied within the project.
With your proposed setup, the drone will still receive the original base coordinates. Could you tell me more about your setup? What coordinate system are the known points based on?
Hello Ruth
What you said makes sense. Since the drone is not “taking measurements in the project” per se, the corrections are not sent dynamically via ntrip. Only in stored points within the project itself in the flow app (would be an amazing feature though).
The majority of my projects are in NAD83 / (State specific plane) and NAVD88 GEOID18.
Currently, if I stake out a target/gcp on the site with a ntrip service (point one, skylark etc) and it checks tight, I am able to simply take an average fix of about 6-8 mins via the same service in a random open area, then store that point in the project. I then turn off the corrections, and use base output in a single fix over that point and connect my smart controller to either local ntrip or caster if cellular service is decent to fly via rtk from the base.
The issue I am having is alot my sites are in remote areas with poor internet connection so I need a way to tie into the gcp without an ntrip service providing corrections, but also not occupy that point as they typically set targets on those points.
If I could offset that gcp and store a new point for my base as a known point, am I correct in thinking that will tie me into the site? Base shift seemed to be the solution however since it is not dynamically sending those corrections, it does not appear to work for my case based off your answer.
Does that make sense?
Firstly, why can’t you set up your base on the targeted gcp? That would be the simpler approach. You can still identify it in the imagery.
Second. You can easily use the Base Shift workflow that you mentioned. All you have to do is collect one more point after you have established the shift and then move your base to that point and in your base settings, use the collected coordinate, which will be tied to the project CS, to send corrections to the drone.
In addition to @dpitman, you can also set up your base over one of the GCPs, establish the RTK link to your rover, collect points with your rover, and then move your base to one of the collected points.
This would work as well instead of using base shift.
