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THE TECHNICAL PAGES
The laser-inclinometer (by Paul De Bie)



INTRODUCTION

I read an article once in Spelunca (June 95) about a laser-distancemeter, that also had a build-in compass and inclinometer. Strangely enough, the inclinometer was a standard analog Suunto! These days, electronic alternatives are available to measure slopes with a much higher precision than the analog Suuntos. In our club, we constructed in 1994 our first home-made electronic inclinometer with a small incorporated laser. Possibly, this was the very first time ever that such a device was build, and we are proud to have participated at what will probably be "the future" of cave-surveying equipment. As a result of our experiences, at least 5 other Belgian surveyors made such a device as well, and everyone is extremely satisfied about it.

Here's a brief description of this device and how to build one.

 

MATERIALS

a) the electronic inclinometer: we used an electronic "level" from Bosch, type DNM6. Precision is 0.3 degrees. Digital (LCD) display. It costs around 3730 BEF (100 USD) + VAT.

b) the laser: an "Infiniter" laser-pointer. Costs around 2000 BEF +VAT (54 USD), but these days you can find them much cheaper. It looks like a ball-point, is made from inox, and is normally used to point out things during slideshows. It produces a powerful laser beam, with a reach of over 100 metres. Class IIIA laser, <5mW output.Digital Bosch level Uses 2 AAA batteries. These days, you can find them in different sizes, materials and brands but keep these things in mind:
- must be powerful enough (some lasers hardly reach 25 m!)
- some small models use expensive Mercury-batteries (button-cells)
- on/off switch must be easy to manipulate or must be replaceable

 

MOUNTING

The level is an aluminium profile of 60 centimetres long! You must shorten it. The measuring-unit is located in the middle of the level, so cut of (with a saw) both extremities of the level, just leaving the central part (about 17 cm. long). It should be possible to really extract the measuring-unit itself, but we did not do this: it is far more easy mounting the laser afterwards, if you leave the surrounding aluminium profile intact! By the way: if you want to make a nice-looking instrument, then cut the edges not at a right angle, but oblique. This way you can re-use the rubber plugs that close off the profile at both sides!

The profile has got a groove on top of it. It is just perfect to lay the Infiniter laser pointer in it, which can be fixed with two small braces (e.g. the ones used to fix electric cable against a wall). I also suggest that you fix a small waterproof switch on it, so you can switch the laser off/on during your measurements. Normally you switch it on by pressing a clip, but then you have to keep it pressed in all of the time. Not very practical!.

In order to protect the LCD-display and the push-buttons, I suggest you put a clear and soft plastic over them, that you fix at the edges by means of tape. The mounted clinometre

IMPORTANT: prior to mounting the laser pointer, you should check if the laser beam doesn't have a lateral deviation. Put the laser pointer on a flat surface (e.g. table) and point the laser beam at a wall which is +/- 5 m away. Now slowly roll the pointer around it's axe (without moving it sidewards!). The red laser spot at the wall should not move. If it does move (or describes a small circle), then the laser beam is not correctly aligned with the body of the pen. You should fix the pointer on the electronic level so that the deviation is in a horizontal plane and not a vertical one. This way, the deviation will not influence your readings.

 

USING THE LEVEL

Keep the level at the "from" survey station, with the tip of the pointer besides the station. Beam at the "to" station. Wait a second or so, try keeping the red spot at the to-station and press the "hold" button on the level. The measured slope will be kept on the display until you press this button a second time. But first, of course, write down the slope in your survey-book. It is a good habit to repeat the measurement again. It should not differ more than 0.5 degrees, if it doesn't: check a third time.

The survey station can be every striking point in the cave: stalagmites, rocks, protuberances etc. But, since finding the appropriate point is often time-consuming and difficult, we use our own surveys stations (and you can even use them with a normal Suunto clinometer as well!). They are very easy to make: take 40 cm of stiff electricity-wire (diameter 2,5 Ó 4 mm▓) and fix a small plastic disc (3-4 cm in diameter) with some tape on one of the ends. To improve the visibility, put some "scotchlite" reflecting tape on the plastic disc. This "lolly" (see picture!) can now be used as a removable survey station: you can easily put it in a mud floor, ply it around a rock etc.! You will save a lot of precious time this way, and you'll always have a survey station there were you want it (often, natural stations or too close to the wall, ceiling or floor to permit an easy and accurate measurement!). Thanks to the scotchlite, you'll even see it at great distances. When using it with the laser pointer, you'll immediately see it when you "hit" the scotchlite!

Of course, you should use and mark a "fixed" survey station from time to time, let's say every 5-8 stations, in case that you have to continue or repeat the surveying in this area of the cave!A removable survey-station

 

ADVANTAGES

To fully understand the advantages of this electronic level, you can best compare with a normal analog clinometer:

- readability: always perfect! Since the reading is stored on the display after pressing the "hold" button, you just keep the level in front of you and illuminate it with your headlight. A normal Suunto is often difficult to read, especially in the dark (and in a cave, it's always dark!). We solved this by mounting a small LED, powered by a small battery, on the glass of the Suunto clino. But the problems remains when the lens of the clino is muddy or damp.

- location of the survey station: of no importance anymore! With a Suunto, your "from"- station can not be too close to the wall or floor. Especially a steep slope UPwards, with a survey station that is too close to the floor, is almost impossible to measure. The poor surveyor has to lie flat on the floor and nearly breaks his neck trying to look up! With the electronic level, this problem does not exist anymore: EVERY point can be used.

- precision: a lot better! The precision is 0.3 degrees. Normal Suunto's are 1 degree. But, more important: the risk of making a mistake when reading the inclination is very small, since the slope is shown digitally. The "classic" error, that one often makes with a Suunto, is reading "6" or "26" instead of "16".

We also have noticed that ALL Suuntos have a problem measuring nearly horizontal slopes. These errors can be in the order of 1.5 to 2 degrees! So, your clino will read +1.5 while it is in reality +0.5 degree. Furthermore, this error is less important or inexistent when measuring steeper slopes. So you can hardly use an error-correction factor in your survey calculations.

Try this with your Suunto: put two wooden poles (A & B) in your garden, 10 meters away, and both +/- of the same length. Now measure the slope from A top B. Suppose it reads -1.5░. Now repeat the measurement from B to A: it should read +1.5, but... there is a fair chance that it reads -0.5 or 0!

- it is robust: ours has survived 10 m drops, without even being de-calibrated.

 

DISADVANTAGES OR THINGS TO KNOW:

- it is bigger and heavier than the Suunto

- as the Suunto, it is not 100 % waterproof (but we have dropped ours a few times in a pool of water and never had any problem with it afterwards. Just open the battery compartment and dry it with a cloth).

- uses batteries (9 V for the level, 2 x AAA 1.5 V batteries for the laser), so there is a potential risk of having a too low battery to continue measuring. The autonomy of the electronic level is very long: you will make at least 50 to 100 survey-trips with it. The autonomy of the laser pointer is however a lot shorter. My experience is that you should replace the batteries every 7 Ó 10 survey trips. So: always carry a set of spare batteries with you, just in case.

- when measuring slopes that are nearly horizontal (<10 degrees), it is not always easy to see if it is a positive or negative inclination. The LCD screen will display the inclination in digits only, but no + or - sign. It will however indicate by means of small triangles what side of the level was the highest (and lowest). Once you're used to it, it isn't a problem anymore. Of course, when measuring steeper slopes, you can tell instinctively whether it is upwards or downwards!
A nice gadget: you can keep the level upside down: the digits on the display will automatically reverse! This is practical when the from station is situated at the cave floor itself.

- you MUST keep the level +/- straight (this means, in a vertical plane, with the LCD display pointing sidewards). If you would put it, in the most extreme case, flat on it's side, it doesn't work anymore. Of course, this is also true for a normal, analog Suunto!! We have tested this and in fact you must really exaggerate (tilt it at least 35 degrees) before the reading gets unreliable.

- Do not point the laser beam in each other's eyes. It should not be harmful, but one never knows.

 

EXPERIENCES

YOU WILL SURVEY A LOT QUICKER! No more hesitation: is it 33 ... or 37 ... or 23?? And after 10 seconds of hesitation (with a Suunto), the lens has gotten damp, and you have to begin again. With the Bosch, you have the reading immediately.

YOUR SURVEY WILL BE MORE PRECISE! We have made several "closed loops" in caves with a nearly perfect closure error in the Z-plane. E.g. a 100 m long closed circuit in Trou Weron, a difficult and narrow cave, with a closure error of X=13 cm, Y=11 cm, Z=0 cm!.

YOU WILL SURVEY LESS FRUSTRATED! Often, with Suuntos, when the stations are badly chosen, or in narrow squeezes, it takes you minutes to make a good reading. You have to lie flat in the water or in the mud etcetera. With the Bosch, these problems belong to the past.

It takes some practise to quickly point the laser spot at the TO-station. But the water-saturated atmosphere in a cave greatly helps: you can see the red laser beam.

You can use the laser pointer to make surface surveys as well, but not in strong (sun)light, and at the condition to make short shots. The laser easily reaches 100 metres or more, but you will not be able to locate the tiny red spot ... Of course, you can survey overnight!

 

CONCLUSION

At a price that is not much higher than the price of an analog clinometer, you can build a device that is a lot more handy and precise. In the hands of a good handyman, who will build it in a waterproof case, it will be even better than what we made. Should a manufacturer of caving gear commercialize this, it would be a lot more compact, cheaper and probably waterproof. THIS IS AN APPEAL TO YOU ALL, MANUFACTURERS!

You could even build in a compass, or combine it with a laser "DISTO" !

Late breaking news (April 1998); DNM 60 L
The model of level that we used has been replaced by another one. The new model is the DNM 60 L, which offers some extra functionalities, such as an "auto-off" to save on the battery  that will switch off the level if it isn't used for a while.

 

 
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