PositionIT Inbox, Oct/Nov 2018

November 19th, 2018, Published in Articles: PositionIT

This month’s letters follow ponder the proposed new national projection and cartesian coordinate system, and address issues around PPK vs ground control points for drone surveys. Send your letters to the editor at positionit@ee.co.za.

Our winning letter: The proposed national projection and cartesian coordinate system

Eric van Duffelen receives his prize from Autobuild Africa's Dave Beatie 2018

Eric van Duffelen receives his prize from Autobuild Africa’s Dave Beatie.

Dear Editor

In response the news that South Africa plans to chance its national projection and cartesian coordinate system: I can see where the Chief Directorate: National Geo-spatial Information (CD:NGI) is coming from. In the 90s to 2000 we all had to gear up for the change from LO to WG, which made a few software developers very rich. The same happened in the late 70s to early 80s when we went from Goldfields to LO. It’s quite obvious that the software developers have more cash power than the whole geomatics profession in total and now want everyone to play by their rules.

So, having had my little rant about this, I contemplated the proposed change and I do agree that it is the right thing to do. Why do we need to confuse our already poorly-educated prospective geomaticians with y, and when they still do not understand x, y, z? Zero south is only for the arrogant old timers, and increasing from the equator southwards is and always will be a massive challenge, let alone increasing west (particularly with negative values getting “bigger” to the east)

You can imagine how bad the conversion was from Goldfields… I still lay awake at night wondering how I survived this.

Now the current problem. We old timers have just managed to convince and educate our engineering and architectural associates that LO is okay, just change the signs of the and values. Why not the z, you ask. Well, we didn’t want to complicate matters threefold. The LO system was based on two-degree zones (what the hell are you on about, you ask again) and us old times then went off the explain the “orange peel theory” – where the smaller the zone the more accurate (I still carry an orange and sharp knife in my pocket just for these occasions). And you ask again: why do we not just base everything on the North Pole and Greenwich? Here we go: you now need to convince them of the massive constants that need to be applied so that your computer can handle all these digits. More frightening still is the debate – who says Greenwich is zero? The French will freak out because they still believe that Paris is zero…

I digress… off to find a beer in a mug with “square shapes” which resemble the various survey related zones… Oh no! What is a conical projection. I just realised as I was looking into this mug that the squares at the lower end are narrower than those at the top – damn Lambert, Gauss and La Borde! The waiter at the local hang out has noticed me freaking out and has just brought me a beer glass (the flute shape). Damn, this is exactly what we are talking about. The shape represents the conical projection. Ha! The whiskey drinkers (aka engineers and architects) get a glass which resembles a spherical projection – now real trouble is brewing.

Can you imagine supplying a drawing to an architect which you haven’t had to convert. Man this is big. Now he may just lose the ability to rotate or rescale. Now how are we going to be able to charge our exorbitant fees to convert his drawing back to a survey drawing for setting out?

My beer, which has just dropped below the equator level, is now warming up as I have just had to help a young chap from overseas with a cell phone and navigation problems who cannot understand why latitude is negative here in Jozi – “is it only Johannesburg?” he asks. My beer hits rock bottom and the waiter is gesticulating frantically to the barman.

So back to the problem at hand.

I agree. This change should happen. I may be able to retire doing transformations in the office with my new (or modified-at-a-cost) software. The three or four degree zones has got to be the way to go – the more zones, the more transformations, the more income. Who cares about the distortion at the edges – this will only apply to pedantic old timers with nothing else to do.

Damn, another beer finished. Just realised as I stood up to go, the earth seems to be rotating in an anticlockwise direction – is this possible or does this just apply when looking north?

Regards
EJ van Duffelen


Re: PPK vs ground control points for drone surveys

Dear Editor

With reference to the article titled “PPK vs ground control points for drone surveys” (PositionIT August/September 2018), I would like to express my serious concern about the concluding remarks and advice, specifically the statement: “This indicates that no GCPs are needed to perform an accurate PPK survey with the eBee Plus drone. This means large areas can now be easily and efficiently surveyed with less time required to prepare GCP marks.”

One of the fundamental principles of sound survey practice is that we need to check all work. This research compared positions from models generated from direct georeferencing (using GNSS airstations computed using PPK) to models using GCPs using one instance of data and demonstrated that it compared within a certain tolerance. It is dangerous to assume that this will always be the case, especially if all the sources of error were not discussed or demonstrated how it was mitigated.

When a PPK survey is done, there are numerous potential sources of error, especially coordinate error, baseline processing errors, and incorrectly applying the geoid. Although I totally support using PPK to compute a priori airstation coordinates, this should be followed by a bundle block adjustment and absolute orientation by tying it to a minimum of four GCPs. If this is not done, it is impossible to prove and verify that the output is referenced to the required coordinate reference system and ensure there is no gross errors.

There is no doubt that direct georeferencing significantly reduces the amount of ground control requires, but to state that it is not required is really bad advice.

Regards
A Parker


Dear Editor

In response to A Parker’s letter: Thank you for your feedback. Every good surveyor should know that survey is about checks. I really think that for the scope of work we conduct (volumetric) drone surveys we can confidently survey our sites with only using post-processing kinematics (PPK), as we always put out check marks to make sure that the deliverables are accurate.

We have conducted numerous surveys (40 000 ha) on Vale Moatize Coal Mine in Mozambique. This was not the first instance that we did comparisons between PPK and ground control point (GCP) markers, it was simply something I wrote up because we tested it with the ComNav system. We have weekly survey data where we compared normal GCPs, PPK and laser scanning surveys of open pit excavations, stockpile areas, run-of-mine (ROM) areas and tailing storage areas. We also tested the different survey methods on areas and stockpiles that didn’t change, and all the results we achieved from the different survey methods were acceptable.

We simply stated that it was possible for us to achieve excellent survey results using only PPK methods, and didn’t give anyone any advice on doing it or how to do it, merely pointing out our experience with that survey method, as we tried, tested and perfected the workflow over a long period of time until we succeeded in getting satisfying results.

Regards
L Potgieter