Quote:

unless it was military grade gps it was only good to a 20' diam

THAT ended in the Clinton administration. A decent quality GPS with SBAS/WAAS can routinely get down to 2.5 meters. Given the time required to dig the tunnel, they would have had multiple position fixes over that time. There are also algorithmic approaches that can get positioning accuracy better by using a fixed base station as a pseudo-reference; RTK GPS which farmers use can get down to centimeter accuracies, as can multi-band GPS using phase differencing.

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sure.... but none of that stuff will work underground.... and certainly they had no access inside the walls.... if i believe you about the accuracy of the devices then that answers that.... but they would need a 30 x 30 point of reference... to essentially 3 decimal places and then some way to " see that from underground as they dig... thats what i don't understand... its not like they set up surveying equipment above ground and inside the walls...

Why not? A new theodolite goes for under $40K. Drug cartels can engineer and build their own submersibles. Not much can't be done with nearly limitless funding.

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it not the cost of the equipment... they spent millions to do this.... its the access. the area around the prison is a flat featureless plane. the reports are that no one was allowed in that area and it was all under the highest surveillance. this was the max hi security prison in all of mexico... thats why they had to start a mile away.... no... nothing was done on the surface it was all underground somehow and thats what has me stumped. and its never been discussed HOW they did it. back to the accuracy for a minute.... you talk about centimeter accuracy... if we are talking decimals of degrees, are you saying that commercial off the shelf GPS these days is what... 6 decimal place accuracy ? i have a new golf gps and its good for about 2 meters repeatability... but supposing you are right.... they arrived at a place 30 feet underneath the jail cell and then dug straight up to hit a 30 inch x 30 inch square 30 FEET above them after tunneling for what was effectively a mile without any surface reference except what they can see from a mile away and probably satellite imagery.... i just need to understand how thats even possible. if they could map from the surface ? fine... no issues but they can't get within a mile and they can't get inside... seems impossible but they did it...

There should be some sort of award for this quality of work. You start with a site plan survey and a cad drawing of the prison and use AutoCAD.

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nothing even that elaborate, although they spent MUCHO DINERO for the BORING MACHINE. And, they bribed some number of prison staff, so they clearly could have gotten exact GPS location of El Chapo's cell.

https://www.businessinsider.com/infographic-of-el-...

According to the article, the cartel spent $50M, not 5, so lots of money well spent

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a cad drawing that tells you " as designed " but not " as built " ? a remarkable gps reading to decimal accuracy of a 30 inch x 30 inch shower that doesn't see the sky but then located under ground with gps that , just for the record, also doesn't se the sky ? all with no surface access or references other than what can be observed from a mile away ? well... i guess el chapo didn't hire anyone from around here. maybe its just me but i see this as a remarkable technical achievement and i was hoping for so genuine high level technical insight. there is something to be learned from these guys...other than how to make 20 billion selling cocaine... its two distinct problems.... lets say the magic coordinates could be had to that accuracy of the showers stall somehow... how do you navigate with that accuracy 30 feet underground with no access to surface readings ?

Lidar and laser linear and angular measurements work quite well in tunnels, to 1mm accuracies. All you need to know is the starting and ending coordinates. We built 5 totally underground petroleum storage sites 1-2M m3 per site and two separate pipeline tunnels 18 and 22km long with only entrance and exit coordinates by GPS. Multiple access tunnels to up to 22 800m long tanks. Multiple level facilities, living quarters, control rooms, power station, pump and filter rooms all a minimum of 50m under the surface.

One of them is here 18.54°N 42.52°E You can see some entrances, some are fake, and associated surface facilities.

https://geo-matching.com/content/the-use-of-lidar-...
http://www.riegl.com/uploads/tx_pxpriegldownloads/...
https://water-serv.com/map-lidar-mapping-of-prittl...

I think there is too much credit given to the men or man who dug the tunnel. A compass along with a sketch of the area and of the prison features may have been sufficient to get your bearings before and while digging.

They spent several million dollars, more than enough to pay for a decent surveyor with a total station. They had to get within 5 ft over just about a mile, so 1 millirad angular accuracy, well below what's achievable with compass

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