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Drawing of Accurate Cave Surveys

by D.J. Irwin

Note:    The term accurate cave surveys covers the range of CRG grade 5–6 and the requirements of the M.S.C.

Once all the calculations have been checked and the errors distributed (assuming the inclusion of closed traverses) drawing of the ‘master’ survey can begin.  The all important point that a draughtsman has to remember, when producing cave surveys, is that all users should be able to understand it with the minimum of effort.   It must be remembered that most cavers cannot read a drawing or survey with the same ease as the surveyor and draughtsman who will have lived with the survey from commencement in the cave to the finished print.  This means that considerable thought must be given to the general layout and presentation ensuring that the finished survey has a clean appearance and be free from any form of cluttering.

Caves are unfortunately not simple geometric forms than can be represented with one or two views, but complicated forms with passages and chambers lying above or below other parts of the system.

There are five basic viewpoints of any cave system that can be drawn to give a complete picture. These are: -

1.                  Plan. This shows the cave as though viewed from above.

2.                  Elevation (projected).  The view point being from some convenient point at the side of the system.  The best position obviously lies on the north – south axis or the east – west axis because the co-ordinates would normally have been calculated form these datums.  Should the cave be best shown from some other datum then the co-ordinates will have to be calculated to suit the required projection.

3.                  Elevation (extended).  There are many instances where the cave passages form large loops at similar levels e.g. Swildons, Cuthbert’s and Eastwater.  To produce projected elevations of this type of system would, unless carefully planned and very well drawn, cause unnecessary confusion.  The alternative is to draw the elevation in which its length is equal to the passage length.  This is also known as the ‘developed’ method.  Although this throws the chambers and passages out of their true relationship at least it will clarify the elevation.

4.                  Transverse Sections.  Where the cave is complex with many levels crossing each other, several sections that are cut across the cave will help to clarify the position.  This section will show the relationship of one level to the other and will lie at approximately at 90o to the projected elevation.

5.                  Passage Sections.  A large number of sections drawn at right angles to the run of the passages will be required to show the local change of passage shape.

The accurate outline survey is primarily of use by the ‘specialist’ caver (1) who will add his own notes onto the survey on whatever subject he may be studying.  In other words he will require a survey of the greatest accuracy that the conventional instruments will allow.  Also he will not want the survey cluttered with floor details, stalagmite deposits and other general data that will leave him no room for his own notes; basically the survey will show passage shape and direction and little else.  The caver will want a map of the cave to enable him to plan his route through the cave and not be terribly worried about the accuracy of the views.  The accurate outline survey can be adapted for this purpose by tracing one survey from the other.

Although the cavers survey will then be up to the same accuracy as the specialist survey it will fall into the definitive descriptive map (2).  Methods of drawing descriptive maps are to be found elsewhere (3).

Summary of Drawing Outline Surveys.

The drawing of any accurate cave survey follows the same pattern whatever the scale and complexity of the system.  After calculating the figures a scale at which the survey has to be drawn has to be decided.  If the size of the system is known prior to the commencement of the survey in the cave it is as well to decided on the scale of the survey then.  This will dictate the amount of detailing that is required.  With the aid of the sketch drawings used to check the calculations the arrangement of views can be determined.  The layout of the survey and its general finished appearance must be given considerable thought before drawing actually begins.  A grid is drawn onto the drawing paper which will be the basic framework to locate the co-ordinates and also will be the reference datums for the description of the cave.  A large cave system (or complicated cave system) might well require several sheets to show all views.  Another important point that must be decided before drawing actually commences is the method of reproduction – now hold your horses not that type of reproduction but whether you intend to use dyeline, gestetner, offset-litho prints for final issue of the survey.  This will dictate, to some extent the actual thickness of line and size of lettering – particularly if the original is to be photography reduced.

Published currently with the survey are two basic reports: 1. Descriptions of the cave with historical notes and 2.  Full details of how the survey was prepared.

Scale

To ensure that the finished drawing of the cave is clear to the user it is obvious that considerable thought has to be given to the scale at which the various views are to be drawn. The co-ordinates should be inspected and the extreme values of the northings and eastings obtained; these will give the outside limits of the cave in the four basic directions.  Should a more detailed picture be required to determine the actual shape of the cave then the piecing together of the sketch drawings used for checking the calculations will provide an approximate answer.

If the cave has extreme co-ordinate values of +560Ft. east, -380ft. east, and 1020ft. north say from the entrance, then the area covered by the cave = 940ft. east – west and 1030ft. north – south (4).  The distance from east – west is arrived at simply by adding the two values of the eastings and ignoring the signs which are purely directional.

The problem is now “At what scale do I draw this survey?”  There are two basic limitations that the draughtsman has to face: - 1.  Maximum size of paper available and 2. the smallest passage width that the draughtsman is expected to draw accurately.  The answer to the problem will, in most cases, be a compromise.  The smallest width that a passage width of 1ft. can be drawn accurately is (here I am open to argument) 0.05” thus making the scale 1” = 20ft.  If on the other hand one is lucky and the passage widths (or heights) never fall below 5ft. the scale need only be 1” = 100ft.  There are several instances of large cave systems being drawn as so small a scale that the general passage width becomes the thickness of a single pencil line; alongside the main line are passage sections drawn at a much large scale.  This type of survey is of little value to either the specialist or sporting caver and should be avoided unless absolutely necessary however accurate the main line may have been surveyed.  It is better to make use of all the information gathered and extend the survey to several sheets than give half of the picture.  In any case this form of presentation could only fall into the map classification.

Inspection of the figures quoted above: 940ft. east – west and 1020ft. north – south, the paper size for the plan will have to be 47” x 51” and allowing for a frame line and other notes that will have to be added, the smallest sheet size will be about 60” x 60”.  From the co-ordinates the sizes required for the elevations can also be determined. Assume the cave depth to be 440ft.

Projected elevation.  =   51” x 22” (if plotted on north – south line) or 47” x22” (if plotted on east – west line)

Extended elevation. If the main route through the cave is 2,540ft. in length of passage the section will be 127” x 22”.  Depending on the size of the sheet being used will help to determine whether to draw the elevation in one continuous length or break it into several parts.

Passage sections.  The space required will depend on the number of sections that the draughtsman will want to show.  It may be possible to accommodate all the sections on the same sheet as the remainder of the views without cluttering the survey; if not then they must be drawn on a separate sheet.

If however the paper size is the draughtsman limitation and the scale is 1” = 20ft. is the smallest to maintain drawing accuracy then several sheets must be contemplated.  If this is to be the case an additional sheet must be produced showing the cave in its complete form though it has to be reduced in scale and simplified for the sake of clarity.  This reduction should be overlaid with lines showing the extent of each sheet of the large scale survey.

Layout and Presentation.

The final appearance of a survey has to have an immediate appeal to the eye and be easy to understand; surveys that appear complicated, even though they may be, will only get an occasional glance from the caver.

The layout of the drawing should be read in a similar manner to an engineering layout where all the views are placed relative to one another.  For instance if the cave is better shown in elevation from the east side of the plan then the elevation should be drawn to the left of the plan so the imaginary eye lines can be mentally carried across the paper from the plan to the elevation or vice-versa.  Avoid the common fault made by many draughtsmen of having the plan at the top of the paper and the projected elevation running across the lower edge of the paper when, if the view is drawn to the north – south lines, the view should be drawn to the left of the plan. (See Fig. 1).

FIGURE 1.

Another common fault is can lead to a certain amount of confusion, the placing of the views too close together in order to get all the information on one sheet.  The final result will be a jumbled mess that is difficult to understand.

Many of the problems that the draughtsman will be faced with at the start of his cave surveying ‘career’ lessons as he becomes more practiced in the ‘art’ of drawing. Initially it is best to discuss the problems with ‘experienced’ surveyors who will provide him with many ideas and much food for thought.

One of the difficulties that the draughtsman will have to face up to is the amount of information he can afford to add to his survey without cluttering it.  The quantity of detail that he will be able to add will obviously depend of the scale at which he is drawing the survey; too much will clutter the survey and make the general presentation ugly.  On the other hand a survey with no detail or informative notes is of little use to anyone except perhaps the surveyor himself.  Collins has pointed out that the surveyor/draughtsman has to bear in mind at all times the reason for the survey.  Once this is clear in the mind some idea of the detail required can be determined; this may well affect the scale at which the survey is to be drawn.  Again there is no one solution to the problem and only experience will show what balance is required.  Accurate outline surveys on the other hand are much simpler that the descriptive maps in as much that the required detail is limited to passage outline and the essential notes as to the accuracy of the survey etc.

Grid

All maps and surveys are overlaid with a grid that enables the user to quickly locate passage junctions and other places of interest from the written description or route severity diagrams of the cave.  The reference numbers will be found inn the text.  The grid has a two fold purpose, one already mentioned, the other to form the framework around which the draughtsman can work to plot in the co-ordinates when drawing up the survey.  For these reasons the grid must be accurately drawn.

The grid on any survey or map has its origin off or on the S.W. corner and so to keep to convention all cave surveys should be the same.  If the cave has promise of further extension (and which one hasn’t?) then place the point of origin some considerable distance from the cave.  In the case of the new St. Cuthbert’s survey the grid origin lies 10,000ft. to the west and 10,000ft. to the south a point (very near Westbury-sub-Mendip) that takes in any survey made of the resurgence should the cave be found to have open passage that far!  By placing the origin somewhere to the S.W. will avoid the need for negative co-ordinates which if used will increase the chance of error in the calculations.

The quickest and easiest way of obtaining an accurate grid is to purchase sheets of FLAT graph paper from drawing office suppliers.  Remember that paper rolled during or after printing will stretch in the direction off the roll.  It is best to check the paper before purchasing with a rule – it has been found that the error can be as great as ¼” in 9”!

To standardise presentation wherever possible the Mendip surveyors have agreed to a standard 2½ square grid.  This is not a hard and fast rule but a guide as it will be realised that there are occasions when such dimensions are not convenient.

Plotting Co-ordinates

The surveyed line, either in closed or open traverse form, is the basic framework of the cave system that the draughtsman uses to produce the passage outline.  As all the views are being reduced to a number of common planes only two of the three co-ordinates obtained for each station are used.

PLAN    The survey lines are obtained by plotting the northings against the eastings.

ELEVATION (projected).  Plot northings or eastings against height.

ELEVATION (extended).  This view cannot be drawn wholly from co-ordinates.  It is constructed by plotting the calculated horizontal (which are usually summated to each station) against height.

TRANSVERSEW SECTIONS Co-ordinates may only be used if the section lies on either the east- west or north- south datums.  The section is constructed by drawing the projected lines against height.

Method

The co-ordinates have to be converted to fit the cave grid.  If the fixed point, which would usually be at the closing point of the main traverse, in say 10,000ft. north and 10,000ft. east is to say the fixed point for the cave lies 10,000ft. to the north of the grid origin, then the summated co-ordinates are added or subtracted to the fixed value.  (see fig.2)

The values of the heights from the fixed datum can remain as the calculated values but their actual O.D. value can be worked out later when required for entering the permanent survey stations onto the survey.

FIGURE 2

Preparing co-ordinates for plotting:

STATION No.

CALCULATED

CO-ORDINATES

NORTHINGS

EASTINGS

 

Northings

Eastings

 

 

1

0

0

10,000.00

10,000.00

2

8.5

-6.47

10,008.50

9,994.53

3

9.56

-8.53

10,009.56

9,991.47

4

19.66

-.6.53

10,019.66

9,993.67

5

22.86

-27.53

10,022.86

9,972.47

Plan

Station 1 is positioned on the grid to suit the extreme co-ordinates.  This will ensure that whole or part of the plan that is required will fit onto the grid that you have drawn.  Once the fixed datum point is positioned, plotting of all the station points may begin.  Station 2 is located by measuring 8.50ft. above the datum line (best measured from station 1) and draw a horizontal line through this point.  Measure 6.47ft. to the left of the datum line and draw a vertical line up through the horizontal to obtain the point for station 2.  Where these two lines cross is the position for station 2.  Repeat this procedure until all the stations are plotted in.  It will help the plotting if you mark at the end of the grid lines the co-ordinate values at that point (see fig. 3).  All eastings with values less than 10,000ft. will be plotted to the left of the datum line and values greater than 10,000ft. are plotted to the right of the datum line.  The northing values are manipulated in a similar manner.  A lightly drawn line connecting each station point on the correct sequence will prove useful when adding passage details.

FIGURE 3

Elevation (projected)

The simplest elevation is built up on the north-south or east-west lines and for most caves this will suffice.  If however an elevation is required along a line say running N.W. – S.W. then the co-ordinates will have to be recalculated to suit the new projection line. This effectively relates all the bearings to the required datum lines.

The basic method of drawing the elevation is as follows.  Fix the position of the fixed point, bearing in mind the maximum and minimum heights of the cave relative to this point.  To plot Station 2 (assuming the elevation is on the north – south line in this example) measure the value of the northing and plot it against the change in height.  In the case of the projected elevation the north – south line will run from left to right of the paper if drawn on a separate sheet.  If however that is, is being drawn on the same sheet as the plans then, as stated earlier, the projection must, in its correct relationship with the plan; in this case the north – south lines will run parallel with the north – south lines of the plan.

Station 3 is plotted in a similar manner as the previous station by measuring off the northing value against the change on height at the station.  This is repeated for the whole of the traverse. (see fig. 4).

FIGURE 4                                   PROJECTED ELEVATION

It will be noticed that in projecting the passages from the plan, the true lengths will not be shown and that this projection will distort passage lengths and slopes.  However, it has the advantage that all features of the cave are in their correct relationship with respect to each other.

Should only part of the system be required to be shown in this form then the elevation or section should be entitled PART ELEVATION or PART SECTION.

Extended Elevation

Before discussing the method of plotting the survey station points a note on presentation is needed. Extended elevations are really special cases in as much as that the other views are drawn in pre-determined limits, i.e., the extreme co-ordinates.  The development of passages involves the passage length which is a constant value but the various ways of presenting the development can present a problem ‘of which one is the best’.  It is advisable to make several layouts of the extended elevation before adding it to the master drawing to ensure that what is being shown will be clear to the user. In fact where difficult elevations are involved try the arrangement on as many people as possible – preferably the typical caver – if he can understand the layout then use this even though the draughtsman may feel it is not the best method.

In the case of a complicated (from a surveyors point of view) system, i.e. (Eastwater, Swildon’s and St. Cuthbert’s) a projected elevation would be extremely difficult to show clearly the run of the major passage as in many cases it would have to be shown as ‘hidden detail’ (i.e. other passages crossing in front of them) (see fig.5).  In this event the extended elevation is the ideal method to clarify the situation.

The method of plotting the main lines is as follows: - Plot in the fixed datum point.  Draw in the horizontal distance between station 1 and station 2 and plot this against the vertical change in height.  Repeat this for stations 2 and 3 by drawing the horizontal distance for station 2 to 3 and plot against the vertical change in height. Continue with this procedure until the whole traverse (if this is all that is required to show on this elevation) is completed.  If the survey is to be simplified by the omission of minor passages then this should be stated.   This type of elevation ignores passage direction but the true length of the passage is retained.   In other words  –  the bends in the passages have been ignored and the passage has been pulled out straight.  It must also be noted that any side passages or chambers that are shown will not be in their correct relationship with each other – this fact must be stated on the drawing.  An outstanding example of this method in use is Stanton’s extended elevation of Swildon’s Hole.

The extended elevation has the great advantage of being able to straighten out a circular route in a cave and present it in a straight line the ends of which is the point where the elevation has been broken.  Where two passages run between the same points one of these passages will be longer than the other.  If these are plotted in the extended form the shorter is broken at a suitable point and a note to the effect that no passage length has been omitted.  If on the other hand, the shorter passage is best shown in full then the longer must be broken and the length of passage omitted stated (see fig.6).

EXTENDED ELEVATION

PROJECTED ELEVATION

Transverse Sections

There are many occasions when a complete section of the cave is necessary to gather the full picture of the shape and general position of chambers and passages.  Theses will be drawn either above of below the plan depending on the direction that they are being viewed and will be at 90o to the main elevation.  In other words they will run on the east – west line if the elevation has been drawn on the north – south line.  The method of drawing the transverse section will be similar to the elevated except that the eastings will be plotted against the vertical changes in height. This type of section will only show the passage section at that point chosen for the section of the cave through which the section line passes.  If the cave is complex then several views of this type will be required. To throw the passages out more clearly it might be suggested that the passage shape be shaded or even blacked out (see fig.7).

FIGURE 7.

to be continued.