mapping systems

[smartbomb]

greetings all,

just joined. as for my background, i was a padi owsi in the 90's until i burned out. teaching in north florida in the 90's was a wild ride. during that time the shop in tallahassee i worked for arranged a cave 1 class taught by a guy who was a legend. unfortunately he perished in a system in wakulla county. at that time i bailed rationalizing, if it can happen to him, it could happen to me. still cave diving intrigues me to this day.

add to this i am a licensed surveyor/mapper. my background here is almost exclusive to remote sensing, including high precision gps, lidar, some radar and now getting into sonar. i am strong in to sensor integration and application developement with respect to surveying/mapping instrumentation.

what i have been toying with for the past few months is a towed/self propelled 360 degree (on the horizontal axis) multibeam echo sounder. my application is for scour mapping at bridge piers but has a strong cross over into cave mapping. there are tons of issues to resolve, cost being the greatest. the biggest challenge in the overhead environment is the lack of positional updates that surface systems employ(gps nmea to the mbes).

the question i have for ya'll is what level of precision/accuracy does the typical cave map need. how critical is the linear distances traveled (+/- 1', .5'). these are generic questions and the search function is my friend.

who knows, this project could be the catalyst to get signed up for cave 1 again.

cheers
scot

[rchrds]

Very interesting work- if you have questions on how to integrate your sensor, or for an example of a system that worked pretty well (fairly well, depending on which side of the data integration you were on) you might consider talking to Bill Stone or Barb Am Ende, if you have not already- Of course Bill built a similar machine for the Wakulla II project, and I believe that Barb did most of the number crunching after the dives. I believe that most of the linear positioning was solved with a combination of multiple INS modules, backed up with periodic radio location stations, whose stations were found from the surface using a ULF beacon. I also understand that the INS suffered from significant drift, I am sure due to the low rates of angular change inherent to being mounted on a scooter in large passage. Anyway-
To answer your last question, here are the standards as far as they apply to cave mapping. The most commonly referenced standards are the BCRA (British Cave Research Association) Standards, and are as follows:

Grade 1: Sketch of low accuracy where no measurements have been made
Grade 2: May be used, if necessary, to describe a sketch that is intermediate in accuracy between Grade 1 & 3
Grade 3: A rough magnetic survey. Horizontal & vertical angles measured to ±2.5º; distances measured to ±50 cm; station position error less than 50cm.
Grade 4: May be used, if necessary, to describe a survey that fails to attain all the requirements of Grade 5 but is more accurate than a Grade 3 survey.
Grade 5: A Magnetic survey. Horizontal and vertical angles measured to ±1º; distances should be observed and recorded to the nearest centimeter and station positions identified to less than 10cm.

Underwater cave surveys, despite some claiming otherwise, are almost always grade 3 or 4 surveys. Currently the state of underwater compasses, and the difficulties with aiming and sighting them, do not allow for much higher accuracy, and certainly not on a continuous basis. In addition, the use of a depth meter for elevation, which only reads in whole feet does not allow for the required vertical accuracy to produce a grade 5 map. In a few rare cases, a grade 5 survey may be accomplished underwater, but it is highly laborious and time consuming. This becomes rapidly evident when loop closures to multiple entrances have large error values. Vertical error becomes self correcting however, as depth is always more or less absolute, whereas in dry caving your depth can continue to wander with no water surface to continuously correct from, reducing vertical loop closure errors, particularly in large loops.

Jason

[bgillespie82]

If you want to play with it in some local areas let me know.

Ben

[smartbomb]

thanks for the input.

ben at this point its vapor ware. i am at the design stage. for the scour monitoring its almost a no brainer. there is off the shelf stuff that i can tweak for either towed or submerged apps. the overhead environment is where the challenge is.

i was thinking the radio beacon strategy would be usable, wifi location or some loranesque setup. ulf ..why didn't i think of that. from a sheer surveying point of view, cave mapping has traditional surveying beat hands down in terms of fun factor. i will keep you updated as thing slowly progress.
cheers
scott

[Slüdge]

you might consider talking to ... Barb Am Ende

Barbara is Squirrel Girl on this forum.

[bgillespie82]

i will keep you updated as thing slowly progress.

Yes, please do

[BackstageDiver]

Completely out of curiosity, what kind of equipment is usually used to map caves? Is it just knotted line, compasses, and an experienced cartographer to draw the lines? Are there any high tech devices employed like intertial tracking, sonar, lidar, etc? Somewhere in between?

[FW]

One way to get positional updates, and loop closure is using "cave locators" (aka pingers). They send a low frequency radio signal through the limestone, and then you can locate the position from above. I have heard of accuracy of a few centimeters.

[mpoucher]

Something that could be used in conjunction with traditional surveying might be useful. The most difficult information to obtain is accurate passage dimensions. Something that could be run through a passage and have marks at known stations would be useful for collecting sidewall and correlating it back to a traditional compass and tape/knotted line survey.

In particular it would very useful in some of the dark water caves.

[smartbomb]

what i envisions differs from the original application where 180 degree scan would suffice for mapping pier scour and could be pulled off with a multibeam and side scan sonar.

the basic design is a ring of 6 transducers to cover a 360 sweep about the centerline axis with ins/imu, depth, direction and position (releative would work and is the most difficult subsurface). each sounder is capable of 50-200HZ or pulses per second. in theory as you advance in the cave, the ring will generate a 360 scan along the path traveled.

ins/imu drift is the enemy. we have the same drift issues with the ins/imu in our airborne mapping system and a typical hydrographic operation. here the major advantage is the ability to minimize the imu drift by a series of jinks or turns and to use gps to provide periodic updates to the IMU in order to correct for accelerometer and gyro drifts.

with traditioanl lidar/hydro/sar, the imu/ins data is used to smooth/fill gaps between the gps position of the vehicle and the exposures(images, laser point, radar reflection at time x) during times of no gps obsrvations. given the overhead environment, there are no positional data (gps) to help steer the imu/ins. the setup now becomes a system where the navigation is derive stricly by angles provided by a "drifting" imu/ins. it doesn't take very long before the drift creates large errors in the positions. the trick will be developing a means to provide additional RELIABLE positioning underwater, in the overhead confinement of a cave system. in the previously mentioned system, radio beacon data was available and may be the only alternative.

my head is starting to spin! i have really created a mind twister; open cranium insert worms