On the Map
The Paper Chase Gets Real
Part 2: Answers, Please!
By Tamia Nelson
April 8, 2014
Last week I posed a familiar question: "Where am I?" But I didn't answer it. Instead, I invited you to answer a number of questions about a selection of topographic maps. I also promised to answer these same questions myself in a week's time. Well, a week has passed, and here are my answers to the questions that I asked. If you accepted my gentle challenge, you may want to compare your answers with mine.
And what if your answers and mine don't jibe? Well, there are only three possibilities: (1) I'm wrong. (Please tell me — and please tell me why, too.) (2) You're wrong. (I'll give the reasoning behind each of my answers. Use it as a guide in finding your own solution.) Or (3) we're both right. After all, some questions don't have just one answer. (I'm dismissing a fourth possibility — that we're both wrong — as unlikely.)
In any case, there's no reason for us to dither any longer. Let's …
Get Down to Business
First, though, here are a few reminders:
- All the maps below are taken from digital copies of USGS quads.
- Except where noted, all distances and elevations are given in English measure — though now that the Brits have belatedly embraced the system that the Corsican usurper imposed on the rest of Europe, I suppose this should be "Yankee measure."
- North is up, always.
- When no scale has been provided, you won't need it.
It's all coming back to you? Good! Then we'll begin at the beginning, hanging …
We're off! In order to minimize confusion, I've put my original questions in plain text, while my answers are in italics.
- In which general direction does the South Branch of the Moose River flow? How do you know?
My answer: The Moose flows from right to left, and since north is up on this map (as on all the maps I've reproduced), that's equivalent to saying it flows from east to west. And how do I know? Because contour lines that cross rivers always point upstream. Check out the crossing indicated by the arrow under the "B" in "Branch." It points to the right, so east is upstream. (Old hands will probably be mystified by the elongated, lanceolate form of the contour at this crossing point. It suggests that the water in the river is higher near shore than in the main channel — an absurd notion. I'm guessing that this topographic aberration is an artifact. Contours were once drawn by hand, and that hand was guided by a human intelligence. This is no longer the case, however. For my part, I miss the human touch.)
You've no doubt noticed a second arrow identifying the place where the same 1800‑foot contour crosses the Red River. I don't have to tell you which way that river flows, do I?
- What is the map's contour interval? (English measure, remember?) How do you know? And no, you won't need the map legend to answer this.
My answer: Every topographic map identifies the contour interval(s) somewhere on the sheet. Why "interval(s)"? Because some maps use different intervals in different places. (You'll see an example of this later.) But you can usually arrive at the interval simply by inspection. And that's the case here. Just count up between the 2100‑ and 2200‑foot lines on the peak to the north of the Moose. Five contours separate the two, and five goes into 100 just 20 times. Conclusion? The contour interval is 20 feet.
- You're standing alongside the Moose River at the point marked X. You look east. Which bank of the river rises more steeply: the north bank, or the south? How do you know?
My answer: The land rises more steeply to the north. You know this because the contour lines lie closer together there than they do to the south. Closer interval=steeper slope. (NB: If many lines merge into one, you're looking at a cliff.)
Did you have trouble with the first exercise? I hope not. But if you did, don't worry. The next one is sure to go …
- What's the drop at the falls below the second (downstream) dam, and how do you know? (I'm going to stop repeating the "How do you know?" coda from here on out, but that doesn't let you off the hook. If you don't know how you know something to be true, you're just guessing. And guessing isn't good enough.)
My answer: The falls is at least 60 feet high. To arrive at this figure, I first determined the contour interval to be 10 feet, simply by inspection. (Count up from the 300‑foot contour to the west of HILL road. Five contours bring you to 350 feet, so the contour interval is 10 feet.)
Next, I counted the lines crossing the river immediately below the dam. There are seven in all. That tells me that the river drops 60 feet from the first contour to the last. (Do you think I've done my sums wrong? After all, seven times 10 is 70 and not 60, isn't it? Yes, it is. But the first line to cross the river is your starting point. It doesn't count when you add up the intervals, so you're left with only six intervals to tally.)
Why do I say the drop below the dam is "at least 60 feet high," then? Because there's a pretty good chance that the river drops some distance before the first contour line below the dam, and a few feet more after the last. (An eighth contour line crosses the river a little way downstream.) And since contour lines merge in the vicinity of the falls — the walls of the gorge are nearly vertical at this point —estimating the additional drop is pretty near impossible. It could be as much as 30 feet, giving a total drop at the falls of around 90 feet. Whatever the exact figure, it's mighty impressive.
- There are several draws — most likely dry streambeds — on the slope west of the HILL road. Identify them. How many are there?
My answer: When contour lines cross a watercourse they point upstream, right? Well this is true even if the watercourse is dry. In fact, it's true if the "watercourse" in question is a dry gulch. Now look closely at the lie of the land to the west of HILL road. There are three places where the contour lines sport characteristic upslope dimples. Those are draws ("re‑entrants" if you're not traveling on a US passport), and I've marked them with red lines on the map above.
Why would you want to know this? Because draws can become torrents after a hard rain. You wouldn't want to camp in (or near) one. And also because they're often hard going if you're tackling them on shanks' pony. A draw lures you in. It can even look like a trail, especially if you've got a canoe over your head. But then, after you've tripped over the sixteenth cobble in your path, you'll realize your mistake. Too late, of course.
Think you've got contours well and truly sussed? Then you're ready to answer these questions, beginning with …
- Assuming that the intermittent stream shown on the map flows all the way to the gaging station at High Falls during some seasons of the year, what's the total drop in feet between the road and the gauge?
My answer: This time around, you have the contour interval handed to you. But there are a lot of lines crossing the stream. Moreover, the first contour line abuts the road edge, and the last is covered by the gaging station symbol, so it's easy to overlook one or the other. But perseverance pays off. There are 18 contour lines from first to last, so the stream drops 170 feet between the road and the gaging station. (The first line doesn't count, right?) The resulting torrent must be quite a sight after a heavy rain.
Do you wish there were a quicker way to tally the contours? Then you're in luck. Since the drop between any two bold contours on this map is 50 feet, you need only observe that there are three such 50‑foot intervals between road and gaging station, plus 20 feet more. That's 170 feet in all. And you only needed to count up to three! If your computational skills are anything like mine, you'll appreciate that.
- Now for the hard bit: What's the intermittent stream's average gradient (i.e., drop in feet per mile) along the same stretch? (Hint: You'll find it easier to answer this question if you print out the map.)
My answer: Though our example is an intermittent stream that no boater in her right mind would wet a blade in, paddlers often need to determine average gradient when planning a trip. A river's gradient gives you some idea how difficult it might be to run. It's not the whole story, of course. Not by a long chalk. But canoeists in heavily laden boats will probably find that any river dropping more than 10 feet to the mile warrants thoughtful consideration — and careful scouting — while an average drop of 50 feet or more to the mile will challenge even expert boaters in flotation‑filled craft.
So the exercise is worth the effort. Unfortunately, counting the contours is the easy bit. Measuring the distance as a river twists and turns is harder.How you do it is up to you. The simplest (and best) way requires that you use a little wheeled gadget called an opisometer, but few paddlers who weren't part‑time professional map‑makers will own one of these rarities. Failing an opisometer, you'll have to align a strip of paper along the watercourse, pivoting the strip whenever the river bends. Mark the start and finish with pencil on the strip and then use the map's scale to determine the total distance. Better do it twice. The paper has a tendency to slip, and errors accumulate. In any event, I came up with 0.333 miles using the pivot‑and‑slip method; 0.325 miles using an opisometer. (In my experience, the pivot‑and‑slip method tends to overestimate distances, though the difference in this example is trivial.)
Now it's time to crunch the numbers. Divide the total drop (170 feet) by the total distance (0.33 miles) to get the average drop in feet per mile. A pocket slide rule will the do the job in seconds, and it slips easily into a map case. It also doubles as a ruler and a bookmark. But pocket slide rules are now even rarer than opisometers. Which means you're on your own. Anyway, in this example I get 515 feet per mile, though the uncertainty in my distance measurement makes such precision unrealistic. Let's say the true value is likely between 500 and 530 feet per mile, shall we? (You weren't thinking about running the stream in spring, were you?)
You can put your slide rule away now. The next problem is a field exercise, and it will take us …
- To begin with, which side of the dam do you think offers the best way around, and why? (A couple of additional ground rules: For the purposes of the exercise, assume you can trespass at will. And even if you know the Indian well, stick to what you can infer from the map.)
My answer: This isn't going to be easy. The right bank of the Indian is less steep near the dam, and that's good, but a road passes invitingly close to the water on the left. The map doesn't show any paths from river to road, but they may exist. On the strength of the map evidence, therefore, I'd rate this almost a toss‑up, with the odds just favoring a take‑out on the left. The final decision — Portage on the left? Or on the right? — will have to wait till you're on the river.
But… There's more to be learned from the map. The squiggles in the river below the dam indicate rapids, and the fact that two contours cross the water in relatively close proximity suggests that the drop is comparatively steep. (How steep? You can't tell from the information I've given you: Although you can tell the contour interval — 20 feet — by inspection, you'd need the map's scale to determine the average drop.)
That means we're back where we started: Portage right or portage left? The nature of the drop may well be the determining factor. If the rapids below the dam are too difficult to run, you've got a long walk ahead of you, and the road is likely the better bet. That means taking out on the left, even if you'll have a steep scramble at the start.
- Sketch a rough topographic profile (cross‑section) of the river valley along the W – E line. (Here again, you'll find that things are easier if you print out the map.)
My answer: The landscape we inhabit is three‑dimensional. A paper map is not. Some folks can look at a map and "see" the terrain. Others see only lines on paper. But the art of reading the lie of the land from its paper projection can be learned, and sketching profiles is one of the best ways to go about it. The sketch doesn't have to be neat, and — for visualization purposes, anyway — it doesn't matter what scale you use. Here's an example: First, a crude freehand sketch of the profile along the W – E line (plus a little bit more on the west end):
And now a somewhat more careful rendering, in which I employed a horizontal scale transcribed directly from the map to construct a topographic profile:
The ticks on the horizontal axis represent places where contours cross the W – E line; the small crosses indicate the elevations of the contours. Note my occasional errors in plotting (circled in red). Transcribing profiles is a touchy, painstaking business, and it's easy to get so caught up in the mechanics that you lose sight of the big picture. (Forests and trees come to mind here.) There's also a considerable degree of vertical exaggeration evident in both profiles: The terrain certainly isn't as steep as the sketches suggest. But theywill give you some notion of what you'd see from the seat of your boat as you approached the dam, and that's the object of the exercise.
Are you tired of doing battle with moving water? Me, too. So let's spend a little time in lake country, where we can tackle a …
- The red arrows identify two stylized anchors, one enclosed in a circle and one not. What do these symbols mean, and why would paddlers want to know?
My answer: The key to this question lies in the map's key (see below; right‑click to embiggen). Why didn't I make the key available when I posed the question, then? Because anyone who uses USGS quads regularly ought to recognize that stylized Admiralty Pattern anchors superimposed on inland bodies of water identify sites frequented by seaplanes: the anchor‑in‑a‑circle is a seaplane airport, while the anchor‑without‑a‑circle is simply a seaplane anchorage. Either way, canoeists and kayakers should stay alert when passing through or near these areas. We don't expect things dropping out of the sky into our path, after all, but it can happen.
- I've marked four faint, dashed lines with blue arrows. What are these, and do you think you'd want to drive your car down any of them?
My answer: Once again, the key to the riddle can be found in the map's key, but you shouldn't need the key to know that these dashed lines — they're single lines, you'll notice — are trails, not roads. And while some trails might be passable for 4WD vehicles, most will not be. (Many designated foot trails are closed to all motorized vehicles, anyway.)
I should add that paddlers may sometimes find themselves using USGS topographic maps that are a half‑century old, or even older. I occasionally rely on maps that were surveyed and drawn not long after the turn of the last century, when paved roads were a rarity and individual cartographers still signed their work. These maps are masterpieces of topographic art, as well as useful aids to navigation. However, there are occasional inconsistencies between the symbols used today and those used on older maps. This can cause some confusion. The current USGS guide to topographic map symbols can be downloaded as a PDF, and it serves to illustrate my point. You will note that there is no mention of seaplane anchorages or seaplane airports in the modern guide, and that the trail symbol is different from that shown on the older map key reproduced below. So local knowledge remains vital, and each map's key is an important resource.
It's also worth noting that the most recent digitalUS Topo 1:24,000 series adds to the confusion, omitting many of the details and features found on earlier quads, including springs, wells, and (?!) trails. That's one more reason why paddlers may wish to supplement the current quads with earlier editions. According to Wikipedia, the USGS took the decision to wipe essential information off the map "in an effort to protect natural resources and the public at large." You might very well think that this "explanation" is simply bureaucratic bafflegab, but I couldn't possibly comment.
Want another good illustration of the importance of symbol and substance to paddlers and other map users? You need look no further than …
- There are three largish islands on the lake. Which one would be the best place to spend the night?
My answer: You could call this a trick question, I suppose. None of the larger islands shows much promise as a place to camp. The stylized blue tussocks tell you that all the islands are marshy, and the lack of a green overprint suggests that none is wooded. You probably wouldn't even find a place to sling a hammock, though at least you'd never be lonely. In fact, you'd likely have plenty of company.
Was this question unfair? Perhaps. But I framed it as I did in order to illustrate the danger of making unwarranted assumptions — something that's always worth bearing in mind.
- Now, let's suppose that you left your car at a put‑in on the eastern end of the lake, then paddled west, heading around the islands mentioned in the first question. You had the wind behind you for once, and you made good time, but as the day wore on, your bowman started to feel distinctly unwell. (It's probably the cold pepperoni pizza you shared on the drive up.) To make matters worse, the easterly breeze has stiffened into a near gale, turning Evergreen Lake into something resembling the North Atlantic in winter. (That's what it looks like from your seat in the canoe, anyway.)
This is the last straw for your partner. He doesn't want to continue. He wants to go home, and the sooner, the better. The problem? He doesn't think he's up to paddling back against the wind‑driven rollers. So you decide to beach your canoe at the western end of the lake and hike out along the road to the car. (You see the road on the map, don't you?) Then you'll drive to the take‑out to pick up your buddy and your gear.
That's your plan. But what's the best place to take out? And is your plan a good one? NB: For the sake of this question, imagine that you have no more of the map than the bit I've reproduced. The rest of the quad was torn from your hands by a gust of wind. (Bet you'll use a map case and lanyard the next time!)
My answer: The long‑winded setup reflects the uncertainties of the situation and the complexity of the problem. My first impulse would be to head for the little bay just west of the W530T mark. There's a trail leading up from the bay to a road, which seems to offer the fastest way back to your put‑in on the eastern end of the lake.
Unfortunately, appearances (and questions) can deceive. The roadwill take you to the eastern end of the lake, but it swings far to the west first. That's not shown on the portion of the map that I've reproduced, however — and this is all you have to go by. The moral of the story? Backcountry navigators can't afford tunnel vision, and they should always be wary of making assumptions about anything that isn't shown on their map.
That said, would I still opt to take to the road if I had no more than this fragmentary map to guide me? Maybe — if my partner were very ill and the wind were very strong, with no hint of a let‑up to come. If that were the case, I'd jog round as fast as I could, hoping that the road would eventually bring me to the east end of the lake, without taking me too far out of my way in the process. As assumptions go, this one is at least defensible, if only on a balance‑of‑probabilities argument. Given a free choice, however, I'd hunker down and wait for the wind to drop, thenpaddle back to the put‑in where I'd left the car, even if I was the only one fit enough to swing a blade.
A final aside: The W530T mark I mentioned above likely represents the elevation of Evergreen Lake. This is a metric quad, so the elevation will be in meters, not feet. The significance of the "T" suffix eludes me, however. Since the quad is labeled elsewhere as "provisional," the "T" may identify tentative elevations, awaiting field confirmation. That's my guess, anyway. What's yours?
OK. I'm tired out — the jog around the lake was exhausting — and I'm ready to let the water do some of the work for me again. In short, it's time to take a trip down a …
- What does the brown stipple scattered along the riverbanks indicate, and what does it tell you about the character of the river?
My answer: This is a lazy river, meandering indecisively from one side of a wide valley to the other. And every time it changes direction, the resulting helical flow moves sediment (gravel, sand, and silt) quarried from the outside of each bend inward, toward the shallows, where it accumulates over time to form point bars. These appear on the quad as stippled areas on the convex (inner) banks of the meanders. When the water level is right, such point bars make good lunch stops and camping places, but if you're planning to spend the night, you'll want to keep an eye on the sky. A heavy rain in the hills might send enough water downriver to wash your tent right off the bar. Better check to see what the engineers at any dams upstream are doing, too.
- What else can you infer about the river and its valley?
My answer: Well, the river flows from southwest to northeast in this reach. We can tell this from the upstream‑pointing contour lines. And even though we don't know the contour interval — and can't easily determine it from inspection (see below) — the comparatively wide spacing between the contours, when coupled with the river's striking meanders, hints strongly that we're looking at a "mature" stream with a modest gradient. In fact, the picture I get from this map is of a classic river floodplain: broad, flat, and subject to seasonal inundation. A dangerous place to paddle during high water, perhaps, but a pleasant summer drift.
By the way, close inspection of the contour lines to the east and north of Silver Star will reveal thattwo lines cross. But unless you're mapping Escher's waterfall, this should never happen. It implies that a single point has two distinct elevations. I attribute the crossed lines to careless draftsmanship and the confusion created by introducing supplementary contours at closer intervals in the river valley. (The map's legend — which I have not reproduced — gives the contour interval as 40 feet, with supplementary 20‑foot contours in areas of low relief. The 4500‑foot line is one such contour.)
The end is in sight. Bur first, it's time for …
- Find OK Slip Brook and the unnamed stream draining Carter Pond. Which stream is steeper overall? (You'll probably want to print out the map before tackling this one. And no, you don't need the map's scale to answer the question.)
My answer: Inspection alone should convince you that OK Slip Brook is the steeper of the two. It is roughly 20 percent longer than Carter Pond outlet — you can determine this by direct measurement, without knowing the map's scale — but while the stream draining Carter Pond drops a bit more than 260 feet in its descent to the Hudson, OK Slip Brook drops at least 360, more than one‑third again as much. The bottom line? OK Slip Brook is steeper.
I suppose I should point out that "steeper" means "has a higher average gradient." It does not necessarily mean "has a greater total drop." Confused? Then consider this landlocked example: The town road in front of my house climbs some 60 feet in going no more than 240 feet. In other words, it has a 25% grade. (Highway engineers measure steepness as "rise over run," or percent grade, rather than in feet per mile.) That's steep. But the nearby state highway climbs nearly 200 feet in half a mile. Now 200 feet is more than 60 feet. There's no doubt on that score. Is the state highway therefore steeper? Not at all. Having hauled 50‑pound loads of groceries in my bicycle panniers over both stretches of road many times, I can assure you that although the state highway involves much more climbing overall, the town road is by far the steeper of the two. And the figures bear me out: a 25% grade (60 feet divided by 240 feet) is steeper than an 8% grade (200 feet divided by 2640 feet).
Conclusion? When you compare rivers to see which is steeper, don't look at total drop. Average gradient is what you want.
- We're not quite done with numbers, I'm afraid."X" marks the spot where OK Slip Brook enters the Hudson, and there's another "X" with the letters "BM" appended to it some distance off to the west. What do these symbols signify, and what can you determine from them?
My answer: A broad "X" identifies a vertical control point, labeled with its elevation; the addition of the "BM" (for benchmark) indicates that the location of the vertical control point is marked with a "tablet," likely a bronze cap set into rock or concrete. In this instance, the upstream benchmark has a surveyed altitude of 1292 feet, whereas, at only 1252 feet, the other control point is 40 feet lower. And though I haven't reproduced the map's scale — making an exact determination of gradient impossible — you can readily determine that the distance between the two control points is roughly four‑fifths of the distance between Carter Pond and the Hudson, as measured along the course of the outlet stream. It therefore follows that Carter Pond outlet is roughly five times as steep as the reach of the Hudson between the mouths of the two tributaries. But don't be deceived. The Hudson's no pushover.
You can also use the control points to refine your estimates of the total drop between source and mouth of both Carter Pond outlet and OK Slip Brook. And in each instance the added drop comes to just 8 feet (1300 feet minus 1292 feet in the case of Carter Pond outlet; 1260 feet minus 1252 feet at the mouth of OK Slip Brook).
That's it. A job well done, if I say so myself. We're on the map.
Last week I posed a challenge to readers, inviting them to test their knowledge of topographic maps. This week I gave my answers. Seasoned gyrovagues probably thought my questions easy, but any paddler whose experience of maps is limited to the display on a GPS may have found the going a little harder. Of course, it's always possible that I got something wrong, and if I did, I hope you'll set me straight. Then keep your eyes peeled for the next map quiz. The paper chase has just begun.
All of the maps in this article were excerpted from digitized quadrangles produced by the United States Geological Survey. And this bounty is made freely available to paddlers — not to mention hikers, climbers, and cyclists — for the price of a click. Why not use the contact link at the USGS Map Locator website to say thanks?
Related Articles From In the Same Boat
- "On the Map: Testing Time for Paddlers"
- "On the Map: First Steps"
- "On the Map: Traditional Navigation in a Digital World"
- "On the Map: Topos to Go!"
- "Planning a Big Trip: Maps and Dreams"
- "Planning a Big Trip: Troubleshooting a River"
- "Planning a Big Trip: Staying Found"
And Interesting Stuff from Elsewhere on the Web
- "The lost era of the A‑Z map?"
A thought‑provoking article from the BBC.
- USGS Topographic Map Symbols
- USGS Map Locator and Downloader
Aladdin's Cave of Wonders. Free PDF copies of USGS quads. Get 'em while they last!
Plus Three Books That Are Well Worth Reading
- Maps & Compasses, 2nd Edition, by Percy W. Blandford (Tab Books, 1992)
Out of print, but used copies aren't hard to find.
- Be Expert With Map & Compass, 3rd Edition, by Bjorn Kjellström and Carina Kjellström Elgin (Wiley, 2009)
The latest edition of a classic orienteering text.
- Mapping, by David Greenhood (University of Chicago Press, 1964)
First published in 1964, it's still in print half a century later. Amazing!
Copyright © 2014 by Verloren Hoop Productions. All rights reserved.