In the first article in this series I gave and introduction and talked about Tools of Cartography at Introduction to Cartography. In this article I will talk about Surveying.
Field surveying has two parts, collecting data and sketching. First I will cover collecting data. This is made up of points called stations. Stations have names. For example your first survey might have names like A1, A2, A3, A4. The second survey might have names like B1, B2, B3, B4. In this example you would have 4 lines on the first survey and 4 on the second. One line would be A1 to A2. The next A2 to A3, etc. A line is made up of 1 to 2 kinds of angles and a distance. One kind of angle is a compass reading and is in degrees or radians. But most use degrees. There are 360 degrees in a circle.
Professionals divide the degrees into minutes (1/60th of a degree) and seconds (1/60th of a minute). But we simply use 1/4 degree divisions with the Sunnto Compass and Clinometer. With military lensatic compass you would probably only read it to the nearest whole degree. The clinometer is the 2nd measure of angle if you are not working on flat terrain. This is vertical angle. It is +90 degrees (straight up) to 0 degrees level to -90 degrees (straight down). If you and a buddy were to use a plumb bob string and protractor for your instrument you would read it to the nearest degree. One would hold and aim this rig and the other would read it. But with Sunnto you can read it to the nearest 1/4 degree.
The tape measurement is hard to mess up. You have a dumb end of the tape and the smart end. One person holds the dumb end on the from station and the other holds the smart end on the to station and reads the distance. But if you want to use pace count for solo work or less accurate work just make sure you know your pace count well. Know how up hill and down hill affects your pace count. And measured chains or ropes might come in handy for solo or team surveying. If, for example, you are measuring 100′, 200′, or 100yd (300′) for survey lines constantly you might use a chain cut to exact length. If solo surveying, you can nail the chain to a tree which might be your station. Use brass nails or screws if possible to avoid throwing the compass reading off.
If you are reading in feet and inches then the guy with the sketch book recording data can convert on the go to 1/10’s of feet. Simply divide inches by 12. Or actually I’d probably use 100ths of feet in the recordings if I felt like it.
1=.08 5=.41 9=.75
2=.16 6=.5 10=.83
3=.25 7=.58 11=.91
You should check accuracy of shots with compass and clinometer. You do that by taking back azmith and back vertical angle and recording them. You then subtract or add 180 to the back azmith and subtract that value from the front azmith. If in the case of sunnto compass you are more than 1 degree in error you should redo both shots. It might be necessary to let a different person(s) shoot the angles to get a shot to within 1 degree accuracy. Also make sure nothing metal is near the compass such as chains, necklaces, rings, cap lights and batteries.
The clinometer is unaffected by metal or magnetic fields only gravitational fields. Note that if the forward clinometer angle is positive then the back angle will be negative and vice versa. If you add the two values the result will be the error. Or you can take the absolute value of the back clinometer reading and subtract it from the forward clinometer reading. Again with sunnto clinometer if you are off more than 1 degree something is wrong. Redo the shots and again might need to let a different person(s) shoot one or both shots. With both compass and clinometer you may want to record the error in the data book but its not necessary because it can be easily calculated again in the spread sheet for statistical purposes.
In cave cartography we record distance in whole feet or feet and 10ths of feet from station to floor, left wall, right wall and ceiling. We call this up, down, left and right distances. If out in open ground you should record distance from station to ground. At least for now I don’t have any idea why you might want to record left, right or up distances. Unless you are passing under a rock overhang the up distance is not necessary at all. You might be in a forested area with cliffs and boulders and need to record a left or right distance. Or it might be distance left or right to a hillside. If it is infinite then use infinity symbol the sideways 8.
These distances should be at right angles to the survey line going towards the to or far station. Or alternatively they can be at half the angle between two survey lines. In caves the down distance is usually and average distance to floor in the area. For distances to walls it would be the farthest point back where you can reach your arm. Much of the time in caves the ceiling height is just a good guess. In some cases it can be measured if the ceiling is low enough.
Other things to talk about is types of survey lines. Most of the time you may have a single run of lines each end to end. But sometimes you need to branch. A branch point can be a run of two or more lines and branch yet again. The end result can look like a tree pattern or some other natural design. If the branch is only a single line its called a radial shot. And one station might have several radials. The end result is that you need this graph of lines in which to draw the sketch around and to draw it to scale. Another topic is in completing loops. This is where you tie one branch back into another branch. It can be tricky and sometimes short sections need to be re-surveyed.
Marking survey stations may be important. If not all of them, then at least key stations that you might want to tie into at a later date. If you need to go back to a station that was not marked in the field then you can survey back to it from a known marked station. A station can be a pile of rocks. A nail in a tree. A piece of florescent flagging tape with station name written on it with permanent black marker pen. Or it might be something carved or glued in place. Though I have to say, if its on public lands, then please try to keep it as inconspicuous as possible.
Sketching is a fairly large topic. I will begin in this article and will carry over to the next one. The sketch is drawn on graph paper and its drawn to a given scale. The sketch’s scale does not have to be the same as the final map but many times it is.
What is scale? Scale determines how much detail the map reveals. For example a 1:1 scale map means that a 1’x1′ square in real life is represented by 1’x1′ on paper. 1:2 scale means 1’x’1 of the entity to be mapped is only 6″x6″ on paper. Those scales are not realistic but you get the point.
1:20 is a common cave mapping scale. But in this case it’s really 1:240 because 1:20 on a cave map means 1″:20′ That is one inch to twenty feet. Another common scale for cave maps is 1″:40′ and 1″:50′. How do you determine the scale you need or want in the sketching and the map? It all depends on the needs of those using the information or the purpose of the map. Time and resource constraints will play an important role in this decision.
You may even map at one scale and determine that you must re-sketch at a different scale and redo the map. The survey line data is useful regardless of the scale. Just remember on the sketches for each survey that you need to note the scale. The map of course will have a note as to the scale and a scale bar.
Most of the time if you are holding your field book open with data on the left side and graph on the right side the direction of north is at the top of the graph page. Sometimes you might want to have north to the left or right side of the page because the page is 4″ wide and 8″ long twice as much area is covered from top to bottom. Regardless of how you do it, a north arrow needs to be drawn on each sketched page.
Other topics to cover in the next article on sketching will be drawing near horizontal lines vs drawing lines on greatly sloping land in the sketch. Drawing symbols and a legend. What kind of symbols to use. how to make a ruler and protractor for sketching. Using a ruler and protractor. Drawing in the survey lines. Drawing detail in around the survey lines and stations. Noting data such as time and date of survey. Listing members of the survey team and their positions. Tallying up some stats of the survey.