Archive for the ‘Mountain Carto’ Category

Race To The Moon with Richard Furno, Part 1 (Kelso)

Wednesday, January 21st, 2009

Follow along with Richard’s first hand narration of how historic events shaped the map, the cutting edge science involved in assembling the photographic base material, and the many explanatory notes included on the final design. The wall map is a piece of art, please enjoy :)

Please join me in celebrating the 40th anniversary of the Race to the Moon! Map co-author Richard Furno has allowed me to turn his keynote presentation into a post on my blog illustrating the trials and tribulations of creating this fabulous wall map for the National Geographic Society’s magazine.

This is part 1. Skip to part 2. View zoomable map at National Geographic.

My friend and cartography colleague Richard Furno retired from The Washington Post as of January 1st, 2009. He had a long and productive career first at National Geographic Maps starting in 1963 and afterwards for the newspaper making daily, deadline driven maps for publication in the next day’s newspaper from 1978 to 2008. He was a victim of a changing media landscape and dreery economic times. Read more about Richard Furno.

Richard has been a great mentor to me and we officially honored him last week in the NewsArt department. While he was at National Geographic, Richard worked on The Moon Map and I’d like to share it’s story with everyone around the globe. Because of it’s length, this blog post will be in 2 parts.

NOTE: You may also be interested in my photo essay on Toni Mair-Terrain Artist Extraordinaire.

The remainder of this post is taken directly from Dick’s Keynote presentation. Any references to “I” are in his first person, not mine. Photo and illustrations either (c) National Geographic or variety of  unnamed sources.

While I worked at the National Geographic, no map produced there was so closely tied to events occurring on the country’s and world’s political stage. With each of many developments — a new satellite, a capsule crash, an important space photo — our enthusiasm for or anticipation of actual publication of the Moon map was affected.

The principal author of the National Geographic’s Moon map was Dave Cook, not myself. The history below relates my involvement. Dave was virtually the sole designer. He was also the main researcher and writer of all the material on the map. He established contact with all the phenomenal people who took part in reviewing and editing the final map, many of whom were involved in NASA’s push to the Moon.

After the Moon map was finished, Dave went on to produce the Geographic’s Mars map showing the newly discovered topography as photographed by the spacecraft of the 1960’s and early 1970’s.

When I finished this presentation, I sent it to Dave asking for his edits and urged that he make abundant additions of his own. He said it was “great” but didn’t take the time to add details of his own. I wish he had. This story is far from complete without his part.

Dave and I have been best of friends from 1963 to today and we continue to love astronomy and all things “space-programish”.

1959: Luna 3

In 1959, the Russians sent a spacecraft called Luna 3 into space that looped around the moon and took the first pictures of the… Far Side of the Moon.

There were many Russian space mission that jolted the citizens, and politicians, of the United States from Sputnik in 1957 well into the 1960′s. The Soviets seemed to be able to do anything in space that they chose. These were truly spectacular events.

At the time, the Soviets did not reveal the results of their space missions unless the Kremlin chose to do so. Much of what happened during their space effort wasn’t learned until after 1990. Their secrecy simply enhanced the U.S. drive to beat the Russians to the Moon.

Luna 3’s Far Side Photo

But the photos Luna 3 took were extremely disappointing (above). The right hand two-thirds of the photo is the Far Side of the Moon. The Russians made detailed maps out of this and a few other comparable photos, but they were simply attempts to apply a mass of Russian names to any and all possible features that they could discern. Eventually, only two features here are somewhat clear and their names have stuck. At bottom right is a dark crater with a bright central peak. They named it Tsiolkovsky. The dark patch toward the top was named the Sea of Moscow.

1959: Mercury Program Announced

America’s rollout of its first Mercury space capsule to the launch pad was literally an afterthought. When the US had its first test launch of a Mercury capsule, NASA hadn’t thought of a way to move it to the launch pad. Someone volunteered their flat bed truck. To prevent bumps on the road from being transferred to the capsule, a mattress was placed on the truck to act as a cushion (it’s just noticeable in the picture sandwiched between the capsule platform and the flat bed). Compare this jerry-rigged contraption with…

(Preview) Dec. 16, 1969: Apollo 13 Rollout

Ten years later in 1969 rocket technology had advanced (above). But let’s get back to our story…

1961: Alan Shepard Rockets into Space (Barely)

“Why don’t you light this candle?!!” Alan Shepard on the pad, May 5, 1961

Following the Soviet Union’s dramatic, first manned orbital flight by Yuri Gagarin, the U.S. sent Alan Shepard into space. But Shepard’s flight was suborbital, far less dramatic and a month later than Gagarin’s flight. It lasted 15 minutes, traveled 116 miles high and went 298 miles downrange.

1961: Quest for the Moon

With just those 15 minutes worth of experience in space, President Kennedy made a startling announcement.

May, 1961: President Kennedy Announced We Were Going to Land on the Moon within 8 years!

America was well behind Russia in its space program. The Russians were succeeding in their space missions at an alarming yet. Yet with nothing more that 15 minutes of manned spaceflight experience,  Kennedy made a speech before Congress proposing the moon landing. The speech followed this exploratory memo sent to Vice President Johnson asking if there was any kind of space effort in which the U.S could catch up and beat Russia. President Kennedy wanted to devote “maximum effort” to such a program.

After meeting with NASA administrator Jim Webb and NASA scientists, Johnson came back with their mutually agreed upon idea to go to the Moon. Kennedy then drafted his famous speech. I was still in college but I was getting interested in astronomy and this got me interested in the MOON.

(more…)

Data: ESA Global Land Cover Map Available Online (ESA)

Sunday, January 11th, 2009

ESA’s global land cover map

[Editor’s note: Updated 300 meter resolution global landcover raster provides crisper, better validated snapshot. Available in GeoTIFF format as 1 file or regional (continental-level) compressed files. Thanks Martin!]

Republished from European Space Agency. 
Original post date: 19 December 2008.

View legend. Image previews of the world map (hi-res jpg | tiff). 

ESA’s global land cover map, which is ten times sharper than any previous global satellite map, is now available to the public online from the GlobCover website. It is the highest resolution land cover map that has been completely validated ever released. 

Sixteen experts validated the map using more than 3,000 reference land cover points and showed it had an overall accuracy of 73% weighted by area for its 22 land cover classes. The map’s legend was developed using the United Nation Food and Agriculture Organisation’s (FAO) Land Cover Classification System (LCCS). 

GlobCover legend
  GlobCover legend

The map was generated using 19 months worth of data from Envisat’s Medium Resolution Imaging Spectrometer (MERIS) instrument working in Full Resolution Mode to provide a spatial resolution of 300 m. Data were collected between December 2004 and June 2006.A consortium of specialists worked to process the map, including Medias France, Germany’s Brockmann Consult and Belgium’s Université catholique de Louvain. 

Global validation points
Global validation points
 

These data are useful for many applications, including modelling climate change extent and impacts, conserving biodiversity and managing natural resources. The map has been downloaded by more than 4 000 people since October 2008. 

The GlobCover project is part of ESA’s Earth Observation Data User Element (DUE). An international network of partners has worked with ESA on the project, including the United Nations Environment Programme (UNEP), FAO, the European Commission’s Joint Research Centre (JRC), the European Environmental Agency (EEA), the International Geosphere-Biosphere Programme (IGBP) and the Global Observations of Forest Cover and Global Observations of Land Dynamics (GOFC-GOLD) Implementation Team Project Office.

The GlobCover LC version 2 is available to public through the following ftp server: 
ftp://uranus.esrin.esa.int/pub/globcover_v2/

On the FTP you will find: 
1) The LandCover V2 in greyscale geotiff 
2) The QL – Quality file geotiff 
3) The legend in xls file 
4) The latest Product Description Manual PDM_I2.1

The GlobCover Land Cover product is the highest resolution (300 meters) Global Land Cover product ever produced and independently validated.  The GlobCover Land Cover product is based on ENVISAT MERIS data at full resolution from December 2004 to June 2006.  The GlobCover Land Cover product has been developed in partnership with EEA, FAO, GOFC-GOLD, IGBP, JRC and UNEP.  The GlobCover Land Cover product is labelled according to the UN Land Cover Classification System.

When using the GlobCover Land Cover product acknowledgment shall be as follow: 

Source data: © ESA / ESA GlobCover Project, led by MEDIAS-France 
Image: © ESA / ESA GlobCover Project, led by MEDIAS-France

Vue 7 Pioneer Landscape Creation App Now Open Beta (MacNN)

Friday, December 19th, 2008

[Editor's note: Alex Tait demonstrated Vue as a 3d application cartographers should be migrating to, away from Bryce, for creating 3d map scenes at his 2008 NACIS presentation in Missoula. E-on is starting to beta test a new version.]

Republished from MacNN.

e-on software has released an open beta of Vue 7 Pioneer, an application for creating 3D landscapes. Starting with an empty screen, users can grow trees, add terrain or water, make alterations to the atmosphere, and tweak colors or materials before animating the environment. Objects can also be either created or loaded into the software, and modeled to preference.

Using the virtual camera, creators are able to move anywhere inside the scene, zoom in or out, and change lens type to obtain a desired shot. Vue 7 Pioneer is available for free from the company website; any modules purchased for the open beta should work with the final release.

Natural Earth III – Data for Visualizing Earth from Space (Patterson)

Friday, August 22nd, 2008

[Editor’s note: I am pleased to announce Tom Patterson’s new Natural Earth III. This dataset is optimized for 3d visualization of the earth from space and includes an optimized cloud layer and enhanced colors for broadcast. iPhone and desktop wallpapers are available. Tom will formally present this new dataset at the SoC summer school in Scotland next week. I’m guessing they’ll be a repeat at this year’s NACIS conference as well.. Dig in!]

Republished from ShadedRelief.org.

Natural Earth III is raster map data for creating illustrations and animations of our planet with a plausibly realistic appearance. Using the data requires 3D or mapping software. Legibility is a key feature. Compared to photographs of Earth taken from space, Natural Earth III offers brighter colors, fewer clouds over land areas, distinct environmental zones, 3D mountains, and continuous rivers. Other features include:

  • Interchangeable data files that align precisely with each other.
  • Georeferencing information.
  • Seamless edge matching at 180 degrees longitude.
  • High resolution: 16,200 x 8,100 pixels (80 arc second).
  • Natural Earth III is free and without use restrictions.

Tom Patterson, US National Park Service (Disclaimer)

Data hosted by: Springer Cartographics.

GPS Kit – New software app for iPhone 3G (Kelso)

Sunday, August 17th, 2008

 

Garafa published GPS Kit for the iPhone 3G on August 14th for $10. YouTube demonstration above from Garafa. I’ve spent the weekend experimenting with the app and the accuracy seems comparable to RunKeeper and iTrail which I reviewed last week. However, GPS Kit has better features, though by the time you read this iTrail will have nearly caught up. GPS Kit meets 10 of my 22 criteria for GPS on the iPhone, the same as iTrail but with GPS Kit having a higher price point but with slightly nicer package. 

The three main functions of GPS Kit are:

  • Dashboard
    • See real time information like latitude, longitude, speed, altitude, heading (cardinal direction, degree), and distance
    • User modifiable statistics (including averages) and units
  • Tracks
    • Intelligent data collection intervals: If you move fast, more points, if slow less points.
    • Save your route
    • View on iPhone in app
    • Share with anyone via email in Google Earth (KML) or Google Maps
  • Waypoints
    • Save points of interest 
    • See how far you are from each waypoint. 
    • View on iPhone in app
    • Share with anyone via email in Google Earth (KML) or Google Maps

Features I wish GPS Kit had:

  • Tracks
    • Better way to see live-GPS route tracking (now a several step process)
    • Resume dialog on restart of app after taking call or other interruption ala Distance Meter
    • Screen lock while recording tracks ala iTrail
    • View on live Google map (now just blank screen?) with both satellite, hybrid, and map tiles
    • Save with KML the stats that are currently displayed on screen for that segment of track
    • In KML record GPS accuracy (eg: accurate to 10m, 310m).
    • KML that does not have 3 nodes at each data point
    • Provide altitude readjustment by matching lat/long against SRTM elevations
    • Import tracks via KML or GPX
  • Waypoints
    • View on live Google map (now just blank screen?) with both satellite, hybrid, and map tiles
    • Set custom icon with preset of common icons
    • Import waypoints via KML or GPX
    • Attach photos to location (location is created on capture of photo)
    • See photo waypoints on the map as icons (multiple, not just active photo waypoint)
    • Click on the photo icon in map view get a big view of the photo and edit text description
    • Attach a longer text description to each location, not just the name

Screenshots of GPS Kit:

Other applications on the App Store that provide portions of similar functionality and their prices:

  • iTrail @ $3- in app map of route, plotting of statistics; has a screen lock while recording. Export to GPX and KML. No waypoint support (coming in version 1.3). Read my review.
  • RunKeeper @ $10- Geared more towards fitness activities with stats like “pace”. Needs to upload data to web service (free) to see route on map. Read my review.
  • GPS Tracker @ free – real-time tracking service including lat, long, altitude, speed, heading, and accuracy. Can export to CVS and KML. Requires web service to record and see on map.
  • gSpot @ $2 –  no privacy concerns (not published online for anyone to see as you move with the app on): lat, long, altitude, speed, heading, and accuracy. Can share waypoints via email. Can display waypoints in the iPhone’s mapping application. 
  • PathTracker @ $1 – real time map showing route/track; distance, time, average speed, current location and altitude; save your path and export in GPX and KML. Available in many different languages including Chinese, French, and Spanish.
  • gps Compass @ $2 – real time stats that are user modifiable; multiple languages including German and Japanese. No map.
  • Speedster GPS @ $1 – speed and altitude in real time with history of pervious data points. No map.
  • Distance Meter @ $3- distance, speed, and pace tracker. No map. Altitude and XY shown for current location only.
  • Geopher Lite @ $2 – distance and heading to waypoint. No map. Ability to turn GPS off in certain functions to save battery. 
  • OverHere @ $1 – email waypoint (current location). No map.
  • Altitude @ $1 – altitude with accuracy (but is it Z accuracy or XY accuracy?). No map.
  • Speed @ free – real time speedometer. No map.

All of these applications require an iPhone 3G with GPS for best performance. 

References: 

iPhone Chat.org and iPhoneFreakz.com.

iPhone GPS with iTrail and RunKeeper (Kelso)

Wednesday, August 13th, 2008

This week sees the introduction of RunKeeper and an updated version of iTrail for the iPhone. Both applications make use of the platform’s built-in hardware support for GPS tracking in real time and recording routes for future reference.

Let’s look at how each app performs against my criteria as expanded from my earlier blog post:

  • Live GPS tracking
  • Real-time display of route and current location on interactive map
  • Custom interval precision (how often GPS location is recorded)
  • Storing of multiple GPS tracks
  • Display of stored GPS tracks on map
  • Location tagging (set placemark)
  • Provides real-time feedback on altitude, speed / pace, and distance traveled
  • Provide and record GPS accuracy (eg: accurate to 10m, 310m).
  • Provide “smoothing” / “straightening of the track based on the GPS accuracy
  • Provide altitude readjustment by matching lat/long against SRTM elevations
  • Export to GPX format
  • Export to KML format
  • Import of GPX and KML tracks to “follow” (eg, download a trail and follow along in the field)
  • Import map (via KML/KMZ ground overlay?) and follow along (thus not requiring Edge or 3G connection)
  • Attach photos to location (location is created on capture of photo)
  • See all photos in an album on the map as icons
  • Click on the photo icon in map view get a big view of the photo
  • Attach a longer text description to each location, not just the name
  • Can the app be exited (not be active) and still record the GPS track?
  • Can the app be screen locked while recording to prevent data loss?
  • Battery life (offer ways to maximize beyond 3 hours)
  • Developer support (responsiveness, feature updates)
  • Price

CONS FOR BOTH:

Active App: The apps are limited by Apple to only record while the app is active on the display. If you take a phone call or decide to answer a text or check an email or otherwise switch out of the application it will stop recording. This is fine if you stop moving and then pick up your route. But you must remember to go back to the app and “resume” the GPS tracking.

Battery Life: The iPhone is NOT a dedicated GPS device and, while you can eek out more “GPS time” by turning off wi-fi and dimming the screen, you’ll still only get between 3 and 4 hours track time before running the battery into the ground.

PROS FOR BOTH:

These apps are just the tip of the iceberg for GPS on the iPhone. Both apps are cheap compared to a full featured GPS and should be complemented with other apps like Where to?, Vicinity, Where, Nearby, and Yelp to get the other “nearby points of interest” found on a full service GPS device. I’d purchase both to support the further development and see where apps go :)

Now to the reviews…

iTRAIL
$3 — get on iTunescompany website

iTrail was first to market (it’s been out for a week+ now) and is priced to move at $3. It has been updated to version 1.2 as of Monday morning and version 1.3 is promised as soon as Apple can review it, which lags a week or two behind real time.

Version 1.2 introduces a screenlock similar to Apple’s own screenlock so you can store the iPhone in a pocket and not worry about accidentally canceling the GPS tracking. I had problems with that while testing version 1 on the trail.

iTrail allows the altitude and speed to be graphed against distance. After the track is recorded (or between recording sessions) iTrail allows the route to be previewed on a Google Maps (requires Edge or 3G service, won’t work in the back country). But there is no ability to zoom in or out, pan, etc. Version 1.1 introduced the ability to export the track to GPX and KML formats via Google Docs (free account required).

The GPX can be loaded into your standard GIS software and the KML can be added to your own Google Maps mashup or viewed in Google Earth.

iTrail can be complimented with TrailRunner for the Mac which will import the GPX file and you can perform all sorts of “sports” analysis on the saved tracks.

I occasionally had problems with the track starting to record with a spurious first data point. This can be worked around by starting a track at the trailhead and then waiting a couple minutes and using the reset track button.

The track accuracy was not especially detailed. The new v1.2 adds ability to increase the precision but still it is not as good as RunKeeper.

Overall, iTrail meets 10 of my 22 criteria and is cheaper than RunKeeper.

RUNKEEPER
$10 — get on iTunescompany websit

RunKeeper has managed to generate quite a bit of hype preceding it’s launch Monday. Overall I find it to be more precise in tracking locations. I think it is doing this by sampling the GPS signal with greater frequency (say every 2 seconds instead of 10 seconds). Unlike iTrail, you must publish your “run” (or ride or walk or drive) to the runkeeper.com site to view it on a Google Mashup that is full featured and can toggle between map and hybrid satellite mode.

While RunKeeper is more accurate in recording the locations along my route than iTrail, there is no user-modifiable setting for this. Nor does RunKeeper track elevation (altitude).

Both RunKeeper and iTrail apps should allow an option to calculate the speed and adjust the interval accordingly in real time to achieve optimal data quality or via a set X seconds interval.

RunKeeper excels in track map quality on the web. The Google Mashup there is fully interactive and has nice icons including mileage markers. There is a full run history of all the different tracks. When you click on a different run (the green bars in the bottom screenshot) the statistics show and the map updates. The web site is free now but not guaranteed to stay that way.

RunKeeper does a better job of displaying fitness statistics on the device. It also looks nicer than iTrail. But it’s lack of map display in the app is a serious negative. Their website interface is not optimized for th iPhone.

Also below, a YouTube video demonstrating how to use the app.

Overall, RunKeeper meets only 5 of my 22 criteria and costs more than iTrail.

The same company came out with GPS Twit in July for posting to Twitter with your location


News Cartographers Map Russia-Georgia War (Kelso)

Monday, August 11th, 2008

Summary of Monday’s maps for the growing conflict between Russia and Georgia over the breakaway provinces of South Ossetia and Abkhazia in the Washington Post, New York Times, Wall Street Journal, CNN, and the BBC.

When conflict hits over the weekend staffing often dictates how exhaustive the mapping coverage will be. Tuesday’s maps are proving to be more detailed in:

  1. coverage of war events,
  2. reference locations, and
  3. secondary context such as relief shading, landcover, population density, pipelines, etc

For background, from the Washington Post (disjointed):

Abkhazia and South Ossetia broke away from Georgia in the early 1990s and have formed close relations with Russia. Last week, Georgian forces launched a major offensive that captured the South Ossetian capital in an effort to reestablish central government control; Russian forces drove them out two days later.

Russia escalated its war in Georgia again Monday, sending troops and tanks out of friendly separatist enclaves to stage the first major invasion of undisputed Georgian territory. One armored column seized a town and major military base in the west of Georgia, while another menaced the central city of Gori.

In Washington, President Bush toughened his rhetoric. “Russia has invaded a sovereign neighboring state and threatens a democratic government elected by its people. Such an action is unacceptable in the 21st century,” Bush said.

Read more . . .

Mikhail Gorbachev has a different take:

What happened on the night of Aug. 7 is beyond comprehension. The Georgian military attacked the South Ossetian capital of Tskhinvali with multiple rocket launchers designed to devastate large areas. Russia had to respond. To accuse it of aggression against “small, defenseless Georgia” is not just hypocritical but shows a lack of humanity.

Mounting a military assault against innocents was a reckless decision whose tragic consequences, for thousands of people of different nationalities, are now clear. The Georgian leadership could do this only with the perceived support and encouragement of a much more powerful force. Georgian armed forces were trained by hundreds of U.S. instructors, and its sophisticated military equipment was bought in a number of countries. This, coupled with the promise of NATO membership, emboldened Georgian leaders into thinking that they could get away with a “blitzkrieg” in South Ossetia.

Read more . . .

And now to the maps:

Washington Post (below)
By Gene Thorp.
View original at full size.

New York Times (below)
View original at full size.

Wall Street Journal (below)
View original at full size.

CNN (below)
Interactive map, the red markers click thru to photo and extensive caption.
View original at full size.

BBC (below)
View original at full size.

Google Maps With A Topographic Overlay (Free Geog Tools)

Wednesday, July 23rd, 2008

free geo tools logo[Editor's note: An alternative to TopoZone.com. Their server can lag a bit.]

Reprinted from Free Geography Tools blog (posted there April 14, 2007).

In addition to the standard image overlays of Map, Satellite and Hybrid for Google Maps, the Active Trails website has an additional overlay button – Topo, for topographic maps. At lower zoom levels, the 1:100K scale USGS topo maps are visible, but if you zoom in close enough, the larger-scale 1:24K maps become visible. Comparing Map, Satellite and Topo overlays, there appears to be a good position match between the three. Active Trails also has placemarks for many kinds of outdoor trails across the country (hiking, biking, equestrian, etc.) entered by site users. You can also install a network link in Google Earth that will let you view the trails there, similar in nature to the new Google Earth layer from Trimble.

Addendum: Fixed the Active Trails links (sorry about that). In the comments, MH points out the ACME Mapper 2.0 site, which is a better choice. It displays black-and-white USGS aerial photos, and also lets you print out a copy of the on-screen map with the push of a button.

Terrain-Shaded Relief In Google Maps (Free Geog Tools)

Wednesday, July 23rd, 2008

free geo tools logoReprinted from Free Geography Tools blog (posted there May 8, 2007).

The Geolabels website offers another image option button for Google Maps, along with the standard Map, Satellite, and Hybrid buttons: Relief. This button gives you views of the terrain in an area, shaded by light at an angle and color.

Go to the website, and either zoom in on an area of interest, or enter the name of a populated place in the box at the upper left and click “Go” (if you hit “Enter” after typing in the name, you’re likely to get an error screen). At left, you will get a list of world localities in the database that match that name; click on one, and the map will zoom in on that location (click on the image for a larger view):

You will also get a marker at the location of the name you entered, along with a message box showing latitude and longitude to way too many decimal places. You can zoom in for a closer view:

Or zoom out:

Major rivers and bodies of water, major highways, and populated places are also shown on the map. Keep in mind, though, that the color shading represents altitude, not vegetation, e.g. in the image above, all that green around Phoenix definitely doesn’t represent lush vegetation. Other sites offer contours or topo maps in the Google Maps interface, but the Geolabels website is a nice complement to them.

Modified 8/26/07 to update URL.

Panorama, Peak Identification And Viewsheds In Google Earth (Free Geography Tools)

Wednesday, July 23rd, 2008

free geo tools logo[Editor’s note: This is the website for the “Hey, What’s That” tool featured in the Where iPod app reviewed in this blog last week.]

Reprinted from Free Geography Tools blog (posted there March 14, 2007).

OgleEarth posts about Hey, What’s That?, a website that lets you enter a location and then gives you:

  • A panorama of what’s visible from that location, marking the position of peak geographic landmarks
  • A list of the peaks, and the ability to show their position relative to your location on Google Maps
  • Terrain profiles (elevation versus distance from the location to any point on the map)
  • A plot all the areas visible from that location in red on Google Maps (aka the “viewshed” or “weapons fan”)
  • Contour lines (zoom in for these)
  • Google Earth export of position, viewshed, horizon line and horizon extent. If the viewshed area is large, this might strain the memory capacity of lesser systems.

It uses 30-meter SRTM version 1 data for its calculations, so there may be some quirks depending on whether there are holes in the SRTM coverage for your area. They’re switching over to SRTM version 2 soon, which is better but still not perfect. It only works in the US right now, but according to OgleEarth, they hope to expand coverage worldwide. A cool site! Check the OgleEarth posting for more info.