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Why Aerial Images Are Important


Quite a few people have asked me if I’ve found aerial images on the DeLorme PN-20 important and useful. Yes! Here is why having aerial photography on a GPS can be important (and a big safety feature). First though I’d like to comment on a recent article in the Washington Times which talked about the PN-20.

Aerial Images and Elevation Perspective

Tracey Capen wrote an article which appeared in the Washington Post and discussed the PN-20. Here is part of what was said.

What separates the PN-20 from the crowd is its ability to store and display topographic, aerial, and satellite images–all of which you purchase and download from DeLorme.

Actually, you don’t need to purchase all of those maps from DeLorme. You get the entire USA covered in their Topo USA product. If you want to purchase aerial, satellite, or USGS scanned topos those do cost extra after the $100 worth of free imagery you get with the device. But here is what I thought was more interesting in the article.

While this feature is attractive and uncommon, at this point it’s more concept than practical application. For example, I downloaded images for Yosemite and had great difficulty picking out familiar features (like large, distinctive peaks) on the PN-20’s 2.2-inch color screen. The USGS Quad topographical maps I downloaded were far more useful.

I agree that aerial images are not good at picking out features like mountain peaks. But that doesn’t have anything to do with the PN20, it is just a function of aerial images themselves. Images taken from the air pointed straight down don’t offer any perspective. Only with the aid of shadows or other cues like changes in vegetation can you make out elevation changes.

Consider this image from Google maps showing a section of aerial photography. The image is of great quality, isn’t hampered by a small GPS screen, yet most people probably couldn’t determine that you are looking at a big mountain range.

If you want to view a map and pick out features like mountain peaks, aerial images are not the tool for the job. Topo maps are the tool. As Tracey pointed out topo maps are more useful for looking at elevation changes like finding mountain peaks.

What Good are Aerial Photos?

Here are two examples of where aerial photos can be a huge asset on a GPS device. Earlier this winter I was enjoying a snowshoe hike on a mountain I hadn’t climbed before. Armed with paper maps, a guide book, and a PN-20 I set out on the climb. However about half way up the trail took a turn from where it seemingly should have, away from my desired destination. I followed the trail for a little while, but it became obvious the trail wasn’t going where I wanted to go.

I evaluated the situation and decided it wouldn’t be a huge compromise of safety to bushwhack the rest of the way up the mountain. Here is where the PN-20 and aerial imagery became an asset. The area I was in had a mix of deciduous and coniferous tree growth. The coniferous growth was very thick, and tough climbing. By looking at the aerial images on the DeLorme GPS I was able to see where the coniferous growth was, and avoid those areas as I plotted the rest of my hike.

I could also flip back and forth between the displays of aerial photography and the topo maps. At one point I could see what looked like a clearing from the aerial photography. But understanding the limitations of aerial photos, I flipped over to the topo maps to see the contour profile. Here I could see that while there was a “clearing” it was the result of a rock slide and rock outcropping. Not something I wanted to try and climb up over! The PN-20 and associated aerial photography and topo maps gave me many shortcuts added another layer of safety.

Here is another example. Last summer I was kayaking on a remote lake. We took with us paper topo maps to backup the GPS and plot our course. The lake was extremely big (two day trip) and had several large islands. Looking at the map, we decided on a route to the right of one particular island to take advantage of more favorable winds. Unfortunately while the topo maps on our GPS showed lots of blue (water) to the right of the “island” we realized later that the blue water was nothing more than a thick muddy bog. We couldn’t travel over it in the kayaks and had a nearly impossible portage through the bog. We could have turned around, but that would have added lots of time against a stiff headwind and added several miles to the trip.

After returning from the trip I viewed aerial photography of the area. From the aerial photography I could plainly see that the area was a bog and travel across it by kayak would be nearly impossible. If I could have had a GPS with aerial photography with me, (or had looked at more aerial images before I left) we wouldn’t have wasted several hours going around the “wrong” side of the island.

Aerial and Topo on GPS

So having topo maps and aerial photography on your GPS (in addition to paper backups!) offers a wealth of information. You need to know which tool to use at which time… topo maps for elevation information and aerial photography for vegetation cues. Having both type of information at your disposal can be a huge advantage, especially when trips don’t go according to plan…. and somehow none of mine ever do. 😉

3 Responses

  1. Great info for someone who has never used a GPS in hand but is in the process of buying one for horseback trailriding.
    This was most informative for me.

    Tommy - May 24th, 2010
  2. Another very useful feature of aerial photographs in a GPS is finding trails. I was hiking in the Santa Monica Mountains recently (Southern California) and decided to take a cross-country shortcut to another trail. I was only using the topo map and it did not show the trail I was looking for. After following a ranch road, then a foot trail, then an animal trail, I got stuck like bug in a spider web in thick overhead chaparral. I knew the trail was ahead, but couldn’t see it on the map. When I finally got out, I reviewed the aerials at home and found that I was only 150 ft. from the trail. Had I flipped to the aerial when stuck, I could have saved myself 45 min. and avoided the scratched arms.

    The DeLorme small screen size is actually an advantage in terms of GPS size, weight, and power (SWaP). If you can’t read the screen well, buy some $15 high power reading glasses and bring them along on the trail. You will be amazed at what you can see in these aerials. What matters most is pixels across the screen, not screen size.

    In addition, I find that the DeLorme and USGS topos are not always accurate as far as trail location and landforms are concerned. The aerials on the other hand are the real deal, the basis for topos. If they are registered to the datum correctly, everything is right where you see it. I find that after a trip my recorded track line sits almost perfectly on top of the aerial trails, while not always so well on the topo trails.

    Gary - January 24th, 2011
  3. With practice I found that I could distinguish between coniferous, deciduous forests, scrub brush, grassland, swamp, bog, shallow water. Lots of ‘parallel sheep trail’ roads will show up on an AP but not on a map.

    I’ve also used APs for checking out rapids for canoe trips, and to see what the conditions beside the rapid are like for portagine. (I would do trips that there were no records that I could find.)

    Bushwhacking generally an AP is better. There are more details that are visible to the guy on the ground.

    AP’s have their limitations:
    1. Clouds and cloud shadows can be problematic.
    2. Depending on the season, some features may be absent.

    Sherwood Botsford - May 14th, 2011

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