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Bird's-eye view

Jan 1, 2003
From Ocean Navigator #108
September/October 2000
Stand around a boat show full of electronics exhibits and you'll soon observe that few of our era's amazing navigation tools have more pure "wow" factor than electronic photo maps. Seeing a vessel plotted on a harbor chart-scale aerial photograph alongside the equivalent real chart is startling and intriguing. Everybody wants to see their home harbor. "Hey, there's a bird's-eye view of my marina. Hey, there's what that ledge actually looks like. Wow."

Electronic photo maps lend an air of realism to electronic navigation. Here are two Florida keys as displayed by PhotoNavigator on the left, Photo Region on the right. Note image differences between products. There are even differences between the quilted sections of each product.
   Image Credit: Ben Ellison

Then there are all the questions. Where do these pictures come from? How do Maptech's Photo Regions and SoftChart's similar PhotoNavigator products differ? How does either work with various charting software packages? Are they really useful for navigation?

There is definitely a school of navigators who don't think the photo maps are very helpful. "Professional captains, especially, seem to consider the aerial photos more a gimmick than a true aid to navigation," said Milt Baker, until recently president of Bluewater Books and Charts, America's largest chart distributor. "On the other hand, professional captains often buy photo CDs because they appeal to charter guests."

At the other end of the scale are boaters who are using photo maps exclusively. We don't know of any, but we do know that people in the industry are frightened about the possibility. "Don't call them charts; they're maps!" a cartographer exclaimed to us. Several salespeople who man boat show booths report that occasionally shoppers say things like "those pictures are way better than those confusing marine maps," and some viewers presume that the images are being downloaded in real time from satellites.

In the middle, and perhaps the majority, are navigators who like to have every bit of available information in order to confirm their position and improve their chances of making wise decisions. Photo maps are full of information additional to charts. In tropical waters like the Florida Keys, some shoals appear with more detail and clarity in pictures than in soundings. Note that other shoals don't appear at all, probably because they are covered with dark weeds matching the color of deep water.

If you are navigating among Maine islands in the fog, seeing that a certain island is tree covered or not might help you collaborate the data coming from your GPS. Entering a harbor or marina, the photo maps often give you much more detail about the location of buildings and slips, or at least a second opinion. For instance, were those extra docks in the Key West screen shot recently built or recently torn down? (Using the "chart info" function, we learn the photo is dated 6/96 and the chart 4/98.)

Finally, you have to admit photo maps are fun. Once you're tied up, you can leave your charting software running, and see if you can't get a visitor and passersby to emit a "wow."

The source

The U.S. Geological Survey coordinates the National Aerial Photography Program (NAPP), an interagency effort to photograph the entire country every five to seven years. The pictures are taken on clear days from planes flying at 20,000 feet using black-and-white or color-infrared film. Each frame covers a square five miles on each side and corresponds to one quarter of a standard 7.5° topographical map. Then they are orthorectifiedthat is, given the geometric properties of a map. The electronic form of these photos is called a digital orthophoto quarter quadrangles (DOQQ), and they are available from USGS. They can be viewed on line in a reduced-resolution, grayscale form at terraserver.microsoft.com.

Shooting the photographs in color is more expensive, and the decision is essentially made by individual states that contribute to the NAPP budget. All the New England states (perhaps more frugal than others) have chosen black-and-white pictures. NAPP is running somewhat behind schedule, and most of today's photo maps are based on photos from the mid-1990s.

The resolution of aerial and satellite photography is measured in meters of earth surface per pixel of photographic or digital image. DOQQ's are one meter per pixel; USGS states that you can not make out a deer in a field but you can see a small grove of trees nearby. For years DOQQ's have been used for applications like land and timber management, evacuation planning, surveying, flood analysis, etc. A couple of years ago first Maptech, then SoftChart, realized that they could turn DOQQ's into products useful to mariners.

The products

The raw DOQQ computer files are enormous, and the first concern of Maptech and SoftChart engineers was how to reduce the file sizes to fit in groups on CDs and work with everyday computers. Both companies succeeded, producing small CD regions of good-looking photo maps whose resolution is the pixel equivalent of detailed harbor charts. Both companies also managed to convert the approximately 60% of DOQQ's shot in infrared color into more natural colors. Beyond that, Maptech and SoftChart took significantly different technological approaches to turning the same USGS files into photo maps.

First, a digression on the subject of scaling is necessary here. With paper charts, we properly think of scale as a solid physical relationship. One inch of a 1:20,000 chart equals 20,000 inches of real world. In the digital world scaling is a trickier subject, and even more so when comparing these two formats of photo maps. Digital images have a purported size, but their actual physical size on screen varies with their pixels per inch and the particular display set-up of your computer. Then they can be zoomed out and in by reducing or making up pixels. At any rate, SoftChart photo maps are purportedly 1:10,000 scale but are 127 pixels per inch; Maptech's 1:20,000 photo maps are 254 pixels per inch. While each company claims that its scale is "correct," everyone agrees that the two formats have the same effective resolution.

Maptech Photo Regions utilize a new and proprietary file format based on customized 256-color palettes and a rigorous compression scheme to achieve quite realistic-looking images in relatively small file sizes. SoftChart chose to convert DOQQ's to the 31-color palette and larger file sizes possible with the same NOS/GEO format as used by their digital charts.

Each company's strategy has pluses and minuses. Charting software makers must make a substantial effort to support Maptech's complex new file format, and several report that is difficult to display and manipulate their photos quickly. Most every ECS program could at least display SoftChart's format before the photos were even released, and developers report that they are relatively easy to open, quilt, rotate, etc. This difference in file types explains why the ECS you already own may be more likely to support PhotoNavigator.

Most observers agree that Photo Region's greater color content results in somewhat more natural and detailed-looking images. The navigator will get more information in areas of subtle color transformation like the edges of some shoals. On the other hand, there is general agreement that in some images the increased contrast of the SoftChart images is more informative, especially in poor light conditions.

The two companies also chose quite different ways to package their photo maps, making comparative shopping difficult. A $150 Photo Region CD covers 16% to 34% of a "ChartKit"-size region and includes the complete BSB digital charts and collateral materials like the coast pilot and marine database for that subregion. You may end up buying digital charts you already own. A $50 PhotoNavigator Mini Region covers about half the area of a Photo Region and is meant to be used with a $50 Mini Region of regular SoftCharts. You'll have to copy either the charts or the photos to your hard disk (unless your computer has two CD-ROM drives). Note that neither company yet has full coastal coverage, and some areas (including inland lakes and rivers) are covered by one company but not the other.

Photo Region CDs have another significant attribute not yet mentionedan added set of oblique photos of particularly important coastal features like harbor entrances and large marinas. These are the same photos seen in printed ChartKits and Embassy guides, all of which are now owned and produced by Maptech. The photos are accessed via icons that appear on your charts indicating both the area pictured and the bearing of the photo. There are approximately 60 to 70 of these pictures on every Photo Region CD, and many users find them to be quite valuable. However, programming the icon display is another hurdle to charting software manufacturers wanting to support Photo Regions.

Both photo map manufacturers provide simple viewers on their CDs, but the pictures are most useful when displayed in a true ECS.

ECS & photo maps

There are a lot of features and performance issues between simply plotting a vessel on a photo map and fully integrating these not-really-charts into an ECS. Frankly, none of the ECS packages has yet achieved perfection.

The ideal way to use photo maps is alongside a similarly scaled and oriented regular chart. Some programs like Maptech's Cruising Navigator make this window splitting and scale synchronization a simple one-click command. Quilting, or stitching charts edge to edge, is a somewhat controversial feature when applied to regular charts; it's nearly essential with photo maps. Since they only cover an area five miles on a side and are not centered on particular places, you are quite apt to find yourself trying to look at a harbor where two or four photos intersect. Several products like Nobeltec's Visual Navigator and ChartView are now quilting photo maps, and both Maptech and the Cap'n are planning to add the ability. Finally, you'll want your ECS to always treat photo maps as special and never pick them in any sort of zooming or moving map operation, just as the Cap'n Voyager has prohibited them from its "select best chart" function.

Some of the preceding comments will probably be out of date by the time this article is published; most ECS companies are constantly updating their products. Besides, we certainly would not encourage anyone to select or change charting software on the basis of photo map support alone. We do encourage users to speak up about the features and formats they'd like to see in their favorite product. These electronic charting firms are small, responsive companies, and the squeaky wheel often gets programmed.

We'd be remiss not to mention the more "do-it-yourself" programs like Fugawi and NavPak, which permit users to georeference and use aerial photos from any source. GlobalNav customers include a couple of dredging companies that regularly have custom photos made of particular projects and then use them with NavPak for close-in navigation.

The future

Maptech and SoftChart are both investigating other sources of aerial and satellite photography that may be more current and cover areas outside the U.S. As we go to press, both companies were considering price reductions on their present photo maps. Meanwhile, average computer speeds and storage capabilities continue to grow and will eventually permit the use of higher-resolution photos. The real-time downloading of high-resolution satellite photos is not yet possible, but it is certainly conceivable.

A real question is whether or not navigators will fully embrace photo maps. We suspect they will.

We recently spoke to the captain of a research yacht just before he embarked on an annual voyage to far northern waters. One of this season's destinations is a rocky isle off Baffin Island that was mined for gold by Martin Frobisher's expedition of 1578. Somehow this island has never been charted, but our salty friend has equipped himself with satellite photos of the island that he found at a Frobisher site on the Internet. Navigators love having as much information as they possibly can get.

Ben Ellison is a marine writer, Ocean Navigator seminar instructor, and delivery captain who lives in Camden, Maine.