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Distance Discrepancies Between Digital Mapping Tools, Paper Maps, and Guidebooks

Caltopo Digital Mapping
Caltopo Digital Mapping

Have you ever noticed that the distances computed by digital mapping tools are substantially different from paper maps and guidebooks of the same area?

I’ve been planning a 200 mile backpacking trip and the digital mapping tools I’ve been using, Caltopo and TOPO!, underestimate the mileage by 15% or nearly 30 miles over the course of my journey. That’s a significant difference, equivalent to two entire days of hiking and food.

What causes these discrepancies and which version of the route should I trust?

Guidebook and Paper Map Mileages
Guidebook and Paper Map mileages

Good question. I’m not sure I can give you a definitive answer.

Here’s a simple example. I’ve carefully drawn a line from Imp Shelter to Mount Moriah in the picture below using Caltopo. I zoomed in when I drew the line to make sure that I captured every twist and turn of the trail shown on the digital map. Caltopo reports that this line, which is a segment of the Appalachian Trail, is 1.39 miles long. But I have a paper map and two guidebooks that say it’s 2.3 miles in length. Topo!, another digital mapping program, says the distance is 1.53 miles in length. Both digital tools are significantly under-reporting the length of this trail segment.

Imp Shelter to Mount Moriah carefully drawn in Caltopo, yielding 1.39 miles
Imp Shelter to Mount Moriah carefully drawn in Caltopo, yielding 1.39 miles

One possible explanation is that the map data in the digital tools is historically out of synch with the paper maps and guidebooks. Trails can be very dynamic: they’re often rerouted due to erosion, landowner disputes, or boundary changes.

But, these digital mapping tools are way off and the trail depicted hasn’t moved that much in the past 50 years.

Surely, I can’t be the only person experiencing this issue. Please enlighten me. Why is this happening?

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  1. Your software is probably not making an allowance for hight gain

    • 1) Altitude difference is irrelevant in most cases. For example if you hike 100 meter at 15% you would add 1.1 meter do the distance which is less than 2% (Pythagorean Theorem – if you want to verify – or basic trigonometry). Only for extreme steep situation that would involve climbing would the distance increase be substantial) Note: 15% is fairly steep. You may encounter in normal hiking a max of 25 to 30%. But rarely is it more than 15%.
      2) I observe personally that the “measuring stick” problem may reduce the GPS counted distance by approximately 10% on an extreme winding track. The error rate is based on the sampling rate that your GPS device uses and the speed of your hiking. A GPS will always calculate the distance between two points as a straight line. The human measuring stick is based on your stride length which is probably anywhere between 0.6 meters and 1.1 meters depending on how tall you are and how steep the the track is. On an average track you would rarely encounter more 10% difference.
      3) So the problem in some cases- and that is a guess – is a tendency of declaring a longer distance by the promoters of the sport and us hikers to make us all feel better on our accomplishments.

  2. Yep, I learned that the miles given was “as the crow flies”.

    • This makes sense, but means that digital mapping tools like this become virtually worthless in mountainous regions.

      What digital mapping tools do take height into account?

      Why can these tools generate elevation gain profiles but not include that data in distance computations. That makes no sense!

  3. Sounds like a version of the costal length problem; the length you measure depends on the length of your measuring tool.

    An extreme example: say you measured your route in 1km long straight segments – you would drastically underestimate your distance. Now measure in .5km straight segments – better, but still underestimated.

    Now zoom down to your scale – even if you catch all the twists and turns you can see, if you zoomed in one more level, there would always be more turns you could capture.

    At a guess, the paper/guide distances are taken from GPS traces with points taken about every second. Unless you put is a section of your trace for every second you’ll be walking, your distance will be lower. At that point you might as well take a GPS trace yourself!

  4. I think the digital maps greatly simplify the details of the routes much as Dane suggests. The data is also quite old in many cases. I have seen that myself with some of the VT sections I hiked last year of the AT. Why not hike a section capturing your own data and see how google earth measures the distance compared with your own track. Not sure if your software can import gps data but try those if you can as well. Just keep in Ming GPS will always have some offset so the cords will never be perfect. Measure an out and back for good fun.

    • This is the answer right here. It’s very simple. Compare Halfmile’s PCT tracks with the PCT trail shown on the USGS topo (same topo used by your software). USGS topo will not capture every switchback and just approximates the trail location, often very wrong when compared to GPS tracks. So if you are tracing their approximation to get a distance you will miss on some miles here and there, let’s say about 10%.

      Distances are also always in the crow flies direction and don’t take into account elevation. Otherwise profiles on a map would have a variable x-axis. Your answer is that the trail location you are tracing is not precise.

  5. Philip, would you mind emailing me a copy of that track? For discrepancies around 10%, the coastal length problem Dane mentions is likely the culprit. For the large discrepancy with the Imp Trail, I’d bet it’s elevation related. I made a little program for my apps to calculate distance of GPX tracks, and I’ve been planning on adding elevation gain as a parameter to it… If I could use your track to test it, I could give you a pretty definite answer about what the calculated distance would be if elevation change were taken into account.

    • Sure. I will send it to you.

      Have you found any digital mapping packages other than your own that include elevation data in distance calculations?

      I can’t believe that an entire industry is built on such a flawed computational approach to distance calculation.

      • To be honest, I haven’t really looked into it much. I would imagine Google Earth might be good. I don’t know about Garmin Basecamp. It is a little strange, I agree.

      • Don’t disagree, but elevation changes don’t generally change the distance by much. E.g., a grade that rises evenly 1000 feet over 1 mile adds 94 feet to distance traveled, a difference of less than 2%. [ by pythagorean theorem hypotenuse equals square root of (5280^2 + 1000^2) or 5374 ] That is, rather than 5280 trail-feet you travel 5374 trail-feet as your elevation goes up by 1000 ft). Difference could be greater for grades steeper than 1000/5280, but I don’t see how you could ever get close to a 15% or even 10% (or 5%!) underage. (Result is not changed if you go up and down, e.g., trail that goes 1/2 mile horizontally with 500 ft gain plus 1/2 mile horizontally with 500 ft loss will have total length of 5374 trail-feet.)

      • Maybe you could get to 5% discrepancy or a little higher over short distances. E.g., if you averaged 2000 feet gain per mile reported “crow flies” distance is about 7% short.

      • Whoops, sorry, didn’t read far enough down to see this issue had already been raised before I posted.

  6. Philip, Part of the discrepancy with the Moriah to Imp mileage is due to a major trail relocation in that area since the USGS map was made. The trail now largely follows the crest of the curving SW ridge of Moriah, passing along the tops of those cliffs with the great views. It also wraps around the NW side of the 3460-ft. hump east of Imp Mountain. This is an issue with many trails on digital map packages that use USGS maps – the terrain detail is great but the trail locations are often way off.

    For the Whites, the Trails Illustrated White Mountains Explorer map CD is better because it has GPS’d trail routes from the AMC’s database. It has many of the same features with elevation profiles etc. Would be interesting to see how measurements made using the track tool on that CD would compare to the guidebook measurements, most of which were done with a measuring wheel on the ground by former WMG editors Jon Burroughs and Gene Daniell.

    I’ve seen the mileages come up short on a couple of map projects where the data was obtained digitally. As others have noted, this becomes more of an issue with significant elevation change. Larry Garland, AMC’s cartographer, could provide a detailed explanation. When he goes out GPS mapping on trails, he has a heavy-duty satellite receiver and takes readings every so many feet (can’t remember exactly how often).

    My Garmin 60csx seems to be come in very closely to wheeled guidebook measurements except on very steep trails. On Carter Dome the other day it came in right at 5.0 miles via the evenly-graded Carter Dome Trail.

    Are you planning a WM48 Direttissima?

    • Great feedback Steve. I will give the Trails Illustrated CD a try. As you can imagine, I’ve been using all kinds of different resources including the AMC White Mountain Guide and the WMG Online, which I view as the gold standard when it comes to true mileages in the Whites. However, those two references aren’t that convenient for compiling long routes or elevation profiles so I resorted to other digital mapping tools since my route contains several the off-trail bushwhacks and roadwalks which are awkward to include in a WMG Online custom route.

      In the end, the spreadsheet rules!

      I am most definitely planning a WM48 Direttissima for this year. It will be an awesome backpacking trip.

  7. I imagine all mapping tools will show you distances “as the crow flies”, which means that the closer you pick your points, the more accurate it will be. But it will not take into account the elevation gain (So, if you hike for 1m, while climbing up 1m, it will only give you a 1m total distance, even though you actually walked a distance of 1.41m – a 41% difference).

    Measuring the trail distance using a measuring wheel will take the elevation into account, so if some guidebooks are using these measurements, that will definitely account for some of the discrepancies you found with your mapping tools.

    I am not sure that knowing the “real” distance (the 1.41m) will be more effective, since your own estimated daily mileage is probably calculated based on the “crow” miles You know how to read the map and see which miles are “harder”, and will take longer to hike (you compensate based on your knowledge of the terrain and yourself). So that 20m day in the Whites will be harder/longer than the 20m day in VA, for example.

    I was thinking about taking the PCT measurements, and adding this “real distance” column to it, just to try to estimate my hike. I guess that after I start hiking and get more accurate data about my own pace, it will be more informative than the current distances.

  8. I make maps for a living (GIS) so I understand the frustration with digital tools. A couple of things could be your problem: what datum are *you* using and what datum is the program in? They need to be in sync or at least compatible with each other.

    Another program to try is the online version of ArcGIS. ArcGis.com You can create a free account and can upload .gpx files and .shp files as well as create your own tracks in the map. I’ve found that their base datum is WGS84 (after experiencing datum shifts from uploading data in NAD27).

    Because you will get different lengths calculated depending on which datum you are using.

    Worth a try at least.

  9. I’ve seen significant variations even from my own GPS (Garmin Etrex 20) to WMG Online. For example Mt. Flume from Lincoln Woods 11.16 miles with 3415 ft elevation gain round trip; my GPX track imported into Google Earth is 10.7 miles with 3330 ft elevation gain. Additionally the distances from the “trip computer” on the GPS unit are always longer than the distances once they are plugged into Google Earth.

    One useful trick that I use is exporting the WMG Online tracks into Google Earth. Unfortunately these tracks often “skip” around as you move from segment to segment. I get around this by using the WMGO tracks as the baseline and then make a new track and “trace” over it including my estimated bushwhacking segments. In general I do exactly what you’re doing, plan using several different tools (WMGO, Google Earth, CalTopo, GMap4, etc), but I do tend to stay all digital… of course I haven’t planned any 200 mile trips either.

    • I have experienced the same variation using Google My Tracks on my phone and seeing different results when uploaded to Google Maps. The latter site adjusts the original file and reduces or consolidates segments removing bends, etc.

  10. Regarding the effect of elevation change … I’m pretty sure my arithmetic is correct (I know that Pythagoras was correct)… 3000ft of elevation change would be required to “stretch” every mile of horizontal distance by Phillip’s 15% difference … rugged that trail is (as Yoda might say)

    Steve’s intel about major trail rerouting certainly must account for a lot.

    Regarding GPS tracks … unless the sampling frequency is very high they will certainly suffer from coastal length problems.

    • The 200 mile route I was referring to has over 70,000 feet of cumlative elevation gain (not even counting the down parts) which would explain the extra 30 miles. Maybe the solution is to take the as the crow flies distance + (elevation gain + loss)/3000 to get a better estimate, or just rely on paper maps which tend to be more accurate.

      • If we assume that the 70,000 feet of cumulative gain occurs in a cumulative 100 horizontal miles of the route (the other 100 miles being downhill), geometry says that the horizontal 100 mile distance is only 0.9 miles shorter than the “on the ground” distance (the hypotenuse). Topography really doesn’t make much difference until you have some very, very, steep average slopes (like the example earlier with 5280′ of elevation gain in 1 mile of trail–yikes). I was very disappointed when I first figured that out because is sure SEEMS like you’re going a lot farther!

  11. I have to second Dane’s comments about the Coastline/Ruler problems. I had the chance to remeasure a trip with a 60CSx and an Etrex 20. The Etrex was using the Russian Glonass as well as the American satellites which gave it a much better number of points to fit and presumably a more accurate location.

    The mileage dropped from about 13 to about 10. Every thing but the rulers were the same.
    So I tend to be a little skeptical of GPS tracks.

    You can also get some anomalous points if you’re in a valley or near a cliff because a strong reflected signal with an incorrect timing throws it off.

  12. When I got my first GPS, I anxiously tracked a favorite hike in Big Bend National Park and was disappointed to find it was 13 miles, rather than 17 as listed. After examining the track on screen, I realized the difference was a variant of the coastline paradox. The early generation GPS had a hard time tracking in forest cover and would lose its fix for a while until I came to a place of thinner overstory or paused a while to check the view. There may have been many wiggles on the actual route chosen but the GPS truncated them into straight lines that shortened the read out distance.

    I think the issue is a combination of the sampling scale used and altitude gain/lost. A 1000′ gain over a mile only adds about a hundred feet to the distance, however, when you account for the meanders and minor up and down variations of the vertical profile, the percentage goes up. That’s my understanding, which has a guaranteed accuracy within 15% to 20% range!

  13. I think if anything this shows why you should never rely to heavily on one tool when planning. Best to use a couple different sources for information.

  14. We had the same problem in the Military in gauging distances especially for fuel resupply and how far ground pounders would go before revolting. Maps I learned are just guidelines. Todays Modern maps I have been told, leave out things that are considered by Homeland Security to be of National Importance. I have often seen this on Google Satelite Maps. Just recently in fact I found an entire lake was removed from the Satelite Photo and replaced with a Forest..I had gone to google to Update my 1990 Topo Map of the area. When I hike nowadays I either rely on my Watch, say 10 hours in. Set up Camp, 10 hours back. But since I retired I go until I get tired and rarely carry a watch anymore since I’ve gotten pretty good a guessing the time with a couple of sticks. Cloudy days are a problem. For multi day trips I always carry a spare meal “just in case”.
    I also found on a trip to Southern Georgia with a friend who needed some help that using his Electronic Toy was not very helpful or accurate either. Took us six hours of driving to get to where he was going and it only took me 5 hours to get back using a Map of the State, tha gain of one hour was equal to 70 miles since that is what my cruising speed was. So rely on as many informational sources as you can, even calling up local Trail Clubs in other States for information…and Old Maps Rock! Just like us Old Guys…we know how to do it right…Lol’s…

  15. Being an old guy, another problem I have in hiking is wandering aimlessly. I say I’m checking the view from different overlooks. My hiking partners are usually ready to put out a silver alert after some of my excursions.

  16. I’ve noticed this problem for many years with Topo and even requested that they add a distance adjustment factor, defaulting to 1. You would change it to 1.15 to bump all the distances by 15%. Unfortunately, they never implemented my suggestion, and haven’t maintained the product for several years now.

  17. From the comments above, people seem to be overestimating the trail difference due to vertical gain/loss. I’m a Civil Engineer, not a surveyor, but I think I’ve got this right. My TOPO! distances seem to include vertical gain/loss in their calculation just fine, but it’s not much of a difference in reality. In terrain with 15% relief, the corrected distance is 6.07 miles. Over a 100 mile trail, it’s only 101.11 miles. The example in a post above used 45% grade, which is quite rare for a backpacking trail. Typical grades are small angles, so the diagonal distance is not much different than the horizontal distance. What we really care about as section hikers is the vertical climb, the condition of the trail, and how it’s laid out (switchbacks). The diagonal distance is not important to me, and TOPO! is plenty accurate enough.

    To check out how TOPO! reports distance, I picked areas in the Sierra’s with dramatic relief such as Yosemite, Kings Canyon or Sequoia. Find a dashed Township – Range line. These lines have distinct makings at some of the corners (Txx Rxx E). Pick a good straight base line and it will be VERY close to 6.0 miles horizontal based on accurate surveying. The TOPO! route will report longer than 6.0 miles in steep terrain. If you add up the total vertical gain and loss, and calculate the diagonal distance, it should be VERY close to the reported distance from TOPO!.

    Tip (use the Shift Key when drawing your route in TOPO! and it will make a nice 2-point straight line).

  18. Clarification in the first paragraph in my post above – I realize I didn’t explain the 6.07 miles – it’s the diagonal distance over 6.0 miles horizontal distance @ 15% grade. So the correct diagonal distance traveled is only 0.07 more than “as the crow flies” 6.0 miles. The only time the diagonal distance will be significantly longer than the horizontal is on really steep slopes, not typical section hiking trails. It seems to me that TOPO! is working fine, but I’m going to keep checking it.

    So there has to be a better explanation to explain significant differences between digital map software trail miles vs trail miles reported in paper maps and guide books. I’m thinking it has be a trail relocation or perhaps the ground measurement. The USGS doesn’t seem to have resources to update their topos anymore, and I’ve found plenty of trails mismarked on these old maps. And a wheel is not a great way to measure long trail distances over rocky terrain either.

  19. Maps will underestimate distances due to the “coastline measurement” problem mentioned. GPS’s often overestimate distance due to noise – early Garmin high-sensitivity ones (eg GPSMAP 60) were more subject to this problem, sometimes referred to as “spidering” because when the GPS is stationary noise produces a “track” looking like a spiderweb around the stationary point. Later models have introduced filtering to reduce that, which can produce underestimates. Take 3 GPS’s on a hike and compare them afterwards and you can see some remarkable differences – I have! I create maps from GPS tracks and have obtained the best results by taking along more than one GPS, then afterwards filtering the tracks with a 10m length filter (10m being the approx. accuracy of a GPS) – those distances have generally been within 5% of wheel-obtained ones.

  20. Here’s another difference. Many of the tools will show you the 7.5 minute USGS maps, but most of them don’t let you print at the correct scale!!! Caltopo is the only online source I’ve found that prints maps correctly. NatGeo’s AllTrails (the replacement for TOPO!) certainly doesn’t work that way (they do 1:18,000 or 1:36,000 but not 1:24,000). The desktop version of TOPO! does this, but other sites don’t support this feature. I like that CalTopo will deliver 1:24,000 and give you a PDF at scale for your printer. Very nice. I kinda wish the GMAP4 guy would figure this feature out too.

    • 1+ that. I’ve switched to Caltopo for that reason.

    • Overall, I think CalTopo is getting some very undeserved bad press in this post. I think Cale and Philip are nailing it. CalTopo is a pretty impressive resource IMHO. The owner relies on free maps, so you get what you pay for in that many times the maps are a bit outdated. On the other hand and in some areas where there are more than one map resource available (e.g. in National Forest territories), you can switch between different map layers to figure out if there have been any notable trail changes.

      In addition, the CalTopo software construct seems to have done as good a job as possible in resolving differences in scale for purposes of accuracy in plot location and distance / altitude resolution.

      The fact that the CalTopo owner has gone to this much effort without making us all pay for it is some kind of a gift.

  21. Great discussion! My degrees are in geography and I work for a mapping software company so this topic is of interest. I do far more mountain biking these days than hiking but use several different programs to support my trip planning and routing finding in the field.

    To help with the trail re-route not shown on the USGS topo I would suggest looking at programs that use Open Street Map (OSM). While not perfect (and at times far from perfect) OSM maps are maintained by users and most of the updates are from people uploading their GPS tracks (sorry I don’t have a citation to back this statement up but someone at my company did a report a few years ago). So while the OSM map might not show the re-route now, you could fix the OSM data yourself to accurately represent the current trail.

    I personally use RideWithGPS (http://ridewithgps.com) these days to do most of my trip planning. There are several map options including OSM. You don’t have to draw the route by hand like in Topo, which I was an avid use of since it was originally Sunflower Software in the late ’90s before being purchased by National Geographic. You simply click on the trail/road and click again a ways up the trail and the program connects the dots for you. You can map hundreds of miles in no time.

    From time to time there are issues with the auto route, especially when using OSM data as the crowdsourced edits don’t always connect trail/road junctions properly, thus the software see a dead end. When I feel up to it I have gone into OSM’s online data editor and fixed the junction and within minutes the junction is fixed and available in RideWithGPS! Amazing! Generally (because I’m lazy :) ) I just use the manual draw tools to route through the broken junction and turn back on auto route. The team from RideWithGPS up in Portland, OR have really done a bang up job with this site. I’ve played with most of the online mapping tools like this and I find this to be the best.

    Keep in mind, most mapping tools grossly over estimate elevation gain. For instance I mapped a 30ish mile route that reported like around 8K of gain. When I rode the route it came closer to 6,800 feet of gain. This is the result of the underlying elevation data used in these programs, which is typically 30 meter resolution elevation data. This means that a 90 square meter area is assigned only one elevation value regardless of elevation change within that 90 sq. m. area.

  22. I’ve used NG’s TOPO! for many years. Still do although it is not compatible with my new Garmin GPS. I think there are several reasons for the discrepancy. First topo map programs use a “DEM” file which is a digitized elevation model. Obviously the model does not take in account every foot of gain and loss, other wise the file would be too big to use. Secondly, the trail on the map is never the trail on the ground. Third pedometers are not accurate because they measure steps not the actual stride. Fourth the distance measurements on GPS’ use fixes to calculate distance. Fixes are usually good to about 30 feet. The only accurate way to measure a trail’s distance is with a surveyor’s tape. Mapping programs are great planning tools but one should always assume the trail is longer that it looks.

    • Jeff, the TOPO! Version 4 upgrade adds a lot of Garmin GPS models that aren’t in Version 3. You might still have to download specific drivers for your model. But maybe you have a very new Garmin.

  23. Great discussion – very informative! My thoughts:

    * As others have noted, ignore “elevation gain/loss” when calculating distance – it’s variance is lost within other greater variables (but is very important when estimating time!)
    * The reason mapping software might deliver a different distance measurement than a map is blazingly simple: any tool will deliver a different answer than any other tool.
    * Out west, mapping software often delivers a greater distance than maps, because the latter might have been done by someone moving a little wheel over a piece of paper. Again, depends entirely on what tool/method is being used.
    * When I’m drawing on TOPO, I zig-zag my hand when bushwhacking, giggle it when on a tough trail, and straighten it out when on a road. This flagrantly subjective personal method delivers about as accurate result as possible – over time, one learns one’s personal foibles and adjusts.
    * If someone has walked the route with a professional GPS, definitely believe that!

  24. As earlier stated, the trail lines on CalTopo and other mapping software, even the printed USGS topo maps show a “estimated trail” line. Example: the famous “99 switchbacks” up Mt Whitney. If you look at the switchbacks on the topo maps (USGS, CalTopo or other) you can only count around 58-60 switchbacks. So trying to calculate distance by drawing lines on the “trail lines” on these maps will obviously be inaccurate.

    I always exaggerate my lines or “overshoot” turns/switchbacks when using CalTopo or g-map pedometer (or whichever program you use) to find distance. I find that it matches up better with distances found at trailhead maps or mileage you get from State Park/Wilderness Service websites.

  25. Personally, I don’t need to see every switchback when planning a hike. I just appreciate knowing there are switchbacks and about how much climb is involved. I don’t understand the need for the kind of precision I’m reading about for hike planning. All these mapping tools are capable, IMHO, it’s just a matter of what you like to use, which is often what you’re familiar with.

    • I’m planning a 230 mile unsupported hike without a resupply and carrying just enough food to get me through 18 days. An error of 30 miles does matter to me because I don’t want to run out of food and I don’t want to carry more than I need to.,I wrote this article because these planning programs overestimate the mileage. It matters.

  26. The “coastline measurement” effect has been mentioned several times. I don’t recall it being mentioned that the “coastline measurement” effect applies vertically as well as horizontally. So using the hypotenuse distance over 100 miles will underestimate the actual vertical ups and downs and therefore will underestimate the additional distance due to elevation gains and losses.

  27. I am currently in the Tyrolean Alps and this problem is vexing me. Two points for me: 1 – I am a Luddite so no GPS, just map and compass. 2 – the trails on my map, 1:35k are in reality only approximations.

    Question: is there a rough guide to converting short (1-2km) sections into real distance. Maybe one that can be modified by no. Of contours crossed?

    Thx, Damian

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