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Setting up D_RTK2 on known point
1703 14 2-7 10:37
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OhioDronePerson
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What is the benefit of setting up on a known point?  This device is designed to find its own location, right?

I work for a construction company, and we do have a survey department so I'm able to get them to set some nails for me.  I'm just wondering how it helps, or if it's necessary.
2-7 10:37
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LV_Forestry
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If you don't want to see the buildings wander a few meters between two surveys, it's better to know the coordinates of the point on which the antenna is mounted, then to fill them in either in the RC for RTK, or in post process (PPK).
2-7 11:51
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patiam
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The base sends differential corrections to the drone. They're called "differential" b/c the base calculates the difference between it's just-calculated position and it's previously known postion. Those corrections are much more accurate if the base knows exactly where it is, rather than using a position calculated without the benefit of corrections. Which is why you set it over a known point and enter the coordinates of the point.
2-7 12:37
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OhioDronePerson
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Thanks Patiam, that makes sense.
2-8 03:58
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OhioDronePerson
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I tried using the D-RTK today for the first time.  When I got to the part where I enter the known point's coordinates, I had trouble with the elevation.  It was given to me in NAVD88.  When I entered my elevation into the RC, it wouldn't accept it as it was around 100' off.  I needed to get moving so I just flew the missions, letting the D-RTK decide its own elevation.

I've been working slowly for over a month trying to understand ellipsoidal vs. orthometric height, along with the geoid.  I believe I understand the equation H=h-N.  I am guessing the D-RTK wants the elevation entered in ellipsoidal height?  Is the surveyed height I was given (NAVD88) the same as Orthometric height?  

Incidentally, the geoid height at my location is -98.78', which would have made perfect sense.  




2-20 11:34
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LV_Forestry
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OhioDronePerson Posted at 2-20 11:34
I tried using the D-RTK today for the first time.  When I got to the part where I enter the known point's coordinates, I had trouble with the elevation.  It was given to me in NAVD88.  When I entered my elevation into the RC, it wouldn't accept it as it was around 100' off.  I needed to get moving so I just flew the missions, letting the D-RTK decide its own elevation.

I've been working slowly for over a month trying to understand ellipsoidal vs. orthometric height, along with the geoid.  I believe I understand the equation H=h-N.  I am guessing the D-RTK wants the elevation entered in ellipsoidal height?  Is the surveyed height I was given (NAVD88) the same as Orthometric height?  

Your equation is correct.  

DRTK2 needs coordinates in WGS84 ellipsoidal format.  So the height you need to enter is "h".  

In order not to be mistaken you can look at Google Earth, it indicates the altitude "H".  

How do you find the coordinates of the point?  are you measuring it with an NTRIP receiver or are you using a geodetic landmark?

I ask you the question because if you use a landmark you must not forget the height of the antenna.  I don't remember if the DRTK2 adds the pole height automatically.
2-20 12:06
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OhioDronePerson
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Thank you.  Coordinates were given to me by my survey team.  

I also wondered whether the D-RTK takes its own height into account or if I'm supposed to add the 6'.  I have assumed I'm supposed to add it as I've seen so many people mention the 1.801m.
2-20 12:09
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LV_Forestry
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OhioDronePerson Posted at 2-20 12:09
Thank you.  Coordinates were given to me by my survey team.  

I also wondered whether the D-RTK takes its own height into account or if I'm supposed to add the 6'.  I have assumed I'm supposed to add it as I've seen so many people mention the 1.801m.

I may be writing nonsense but I seem to remember that if the coordinates entered manually differ by more than 50m from the coordinates calculated by the receiver then they refuse to validate them.  Which explains the error you had when putting the geoidal altitude.
2-20 12:18
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LV_Forestry
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OhioDronePerson Posted at 2-20 12:09
Thank you.  Coordinates were given to me by my survey team.  

I also wondered whether the D-RTK takes its own height into account or if I'm supposed to add the 6'.  I have assumed I'm supposed to add it as I've seen so many people mention the 1.801m.

Ok so you need to convert these coordinates to WGS84 decimal.

I have a big doubt about the height to add.  But the best way to be sure is to do your survey by including in it the point that your survey team gave you.  You will quickly realize by comparing the raster with the altitude of the point if 1m80 are missing, or if they are too many.

I'll look in my notes to remember how I was doing.
2-20 12:22
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LV_Forestry
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I added 1.8019m.  Which corresponds to the length of the pole + the distance from the base of the antenna and the center of the phase.

Measure your pole anyway, not sure it's the same.  For the antenna phase there is a label on the side which explains the offsets to be applied.  There must be 141.9mm, it is the one that interests us.
2-20 12:40
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OhioDronePerson
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I appreciate all your input.  I'll do the conversion and add the pole height on the next flight.

What is the 141.9mm you mention?
2-21 04:39
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LV_Forestry
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OhioDronePerson Posted at 2-21 04:39
I appreciate all your input.  I'll do the conversion and add the pole height on the next flight.

What is the 141.9mm you mention?

The 141.9mm correspond to the distance between the base of the receiver and the center of the phase of the antenna.
I found detailed information shared by Patiam on this forum :
NEW D-RTK 2 "User Manual"!!! | DJI FORUM

DRTK2ARP.JPG DRTK2B.JPG
2-21 10:02
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OhioDronePerson
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Wow, thanks.  I just measured it and it's 5'-5 3/8" - or 1660mm.  Adding the 141.9 brings it to to magical 1.8019m I keep reading about.  I would have just accepted the number but it's nice to understand where it's derived from.  

Thanks again
2-21 10:49
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OhioDronePerson
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The processing of this flight is finally done.  When I started to look around the point cloud, the first thing I wanted to do was look at my checkpoint.  Immediately I could see that it was off by a couple feet or more.  Eventually I looked at the point where my D-RTK was setup.  I couldn't see the point itself of course with the D-RTK in the way, but getting close enough I could see that this point was also off by a couple feet or more.  I could see they errors were similar in value, and doing the math showed that I'm only off by .18 and .14 feet in northing and easting for relative accuracy.  Absolute accuracy is garbage though.

So what did I do wrong?  I entered the point into the RC correctly as I checked it twice before hitting enter.  The pole was level, or at least close enough not to be off by feet.

I didn't enter anything into Pix4D, just loaded the images and processed as normal.  Do I need to alter my workflow?
2-23 06:43
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LV_Forestry
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OhioDronePerson Posted at 2-23 06:43
The processing of this flight is finally done.  When I started to look around the point cloud, the first thing I wanted to do was look at my checkpoint.  Immediately I could see that it was off by a couple feet or more.  Eventually I looked at the point where my D-RTK was setup.  I couldn't see the point itself of course with the D-RTK in the way, but getting close enough I could see that this point was also off by a couple feet or more.  I could see they errors were similar in value, and doing the math showed that I'm only off by .18 and .14 feet in northing and easting for relative accuracy.  Absolute accuracy is garbage though.

So what did I do wrong?  I entered the point into the RC correctly as I checked it twice before hitting enter.  The pole was level, or at least close enough not to be off by feet.

is .18 = 0.18ft ? aproximatly 5cm.

This is the precision you can expect from photogrammetry. I would even say it's very good. Now you have to offset the raster in accordance to align the GCP.

You can send it to me if you want to take a look.
2-23 07:27
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