written by Dave aka Team Landshark
The position inaccuracy comes from the way GPS positions are computed. Your GPSr receives signals from the satellites, and measures the length of time it took for the signal to travel from the satellite to the GPSr. It then multiplies by the speed of light to compute the distance between you and the satellite. In the physics world we call this a "time of flight" system. The GPSr does this for all satellites it can "see."
In order to make use of this information, the GPSr needs to know where each satellite was located when it transmitted the data. To do this, each satellite transmits some information about its current orbit. This is called "ephemeris" data. The ephemeris data is transmitted by the satellite only once in a while…IIRC it's about once every 90 seconds. When you turn on your GPSr and it says "looking for satellites," mostly what it is doing is waiting for the ephemeris updates to roll in. The GPSr "sees" the satellites almost immediately, but without the ephemeris data it doesn't know where the satellites are, so it's helpless. On my Oregon 450, the "satellite view" shows a signal strength bar for each satellite the GPSr can see; the solid bars are ones where the unit has received the ephemeris update, while the ones filled in white are ones with no update yet. The reason it can take some GPSrs "forever" to lock in is that if the ephemeris update is garbled due to weak signal, the GPSr has to wait for the satellite to transmit it again. For marginal signals, this process can take a long time to complete. Also FYI, I think newer GPS units have some way of modeling the evolution of ephemeris data, so if you turn off your GPSr for ~a few hours, it "locks in" much faster when you turn it back on. However, if you wait too long (a day or more) or if you get on a plane and go to a completely new location, the ephemeris evolution model breaks down and it has to go back to waiting for ephemeris updates the old-fashioned way.
OK, so now the GPSr knows where all of its satellites are located. It can compute time of flight and thus distance to the satellites. It's a simple matter to solve for the spot where all the distances converge. That is, you want the place in 3-D space where gives you the right distance to all of the satellites.
So where does the error come from? Simple: remember that we multiplied the time of flight by speed of light to get the distance? Well, this is just an approximation. The speed of light is not a constant when you change materials; it depends (mostly) on matter density. Denser materials have slower speed of light. Those clouds overhead? They make light go slower. Those tree leaves overhead? Same thing. Moist air? Same thing. You also get "multi-path" effects, which is the signal bouncing off a building and taking a longer path to get to you. All of this contributes to an error in the estimated distance ***for each satellite***, and a different one for each satellite, at that. So when you go to make that computation of "where do I need to be to make all of these distances work out?", you can't get it exactly right. If there were no such speed-of-light effects, the position accuracy of GPS would be under a foot.
Oh, one more thing to mention. The GPSr can't actually compute the time of flight until the end of this exercise, because while it knows what time the satellite transmitted its signal, the GPSr doesn't know what time it is locally. The computation that gives the position *also* gives the local time. It's all one big hairy computation. The math is ferocious. At the end of the process, the GPSr knows where it is, and also what time it is locally. This means that on average, your GPSr is the best clock you own — typical time accuracy is around 10-20 nanoseconds. FWIW one nanosecond is almost exactly one foot, so if your unit is reporting 15 foot position accuracy, you can assume that implies (roughly) 15 nanosecond time accuracy.
Now for the kids exercise. Start with 5-6 ropes, preferably around 10-15 feet long. Mark a spot on the ground with a rock, flag, whatever, and stretch out the ropes so all the ends meet at the rock/flag, but point them in different directions. Now cheat a little: pull a few of the ropes 8-12 inches away from the spot, and make few "overrun" the spot by the same distance.
Identify 5-6 kids (one for each rope) to be satellites. The rest of the kids are geocachers. Position one kid at the end of each rope. Have them pick up the rope end, and tell them that once they pick it up, they must remain in that spot until you tell them they can move again." Have all the satellites pull in their ropes completely.
Now pick your first geocacher, and give him the end of one of the ropes. Have him walk away from the satellite until it's slightly tight. Explain to the kids that the ropes represent the GPSr's estimate of the distance to the satellite. You can explain about the time of flight thing if you want to, but younger kids probably won't get it.
Tell that first kid that he could be any place on earth that is that same distance from the satellite. Where is he? You may need to prompt him to walk a circle around the satellite, but some kids get it instinctively. Answer: he could be anywhere on that circle. Not very useful. But what if we add a second satellite?
Set up a second kid the same way as the first, but with the rope from a different satellite. Explain that they could be any place that the two signals intersect, because we know the distance to TWO satellites know. Ask them to find the place. Most kids will figure out quickly that there are two such places. If not, help them find the second place.
Now add a third kid the same way as the first two, and ask them to find the location. This time, there is only one place. Point out that the place is not exactly on top of the rock. Why not? Because our estimate of the distance to the satellite is only that: an estimate. It has some error in it, and that error makes for an in where the computed position is. What's worse, with only three satellites, you can't even estimate how much of an error you made!
Add the other satellites, one by one, and repeat. Watch the error get smaller. After each addition, ask the kids to estimate how big a mistake is possible, not by looking at where the rock is, but by looking at how much the ropes overlap or don't touch. You'd be surprised at how good your average group of kids is at making this estimate.
That's it. The only thing left to point out is that they did this walking on the ground which is a 2D object, but the earth is a 3D object. That means you need one more satellite for everything. Four to get a basic position, 5 or more to be able to estimate accuracy.
Comedy King Contest and Event (GC54Z6R)
Be the Comedy King of the Austin area. Place a funny cache for fun, prizes and glory.
This idea was borrowed (stolen) from Mrs. Captain Picard and her sidekick, Footnotes Coach who placed a LOL event in the San Antonio area. Difficulty is higher because you need to place a cache and/or find several others.
Who can place the funniest geocache in the Austin area?
Place a funny cache and WIN A PRIZE at the EVENT on July 12, 2014.
Geocaches can be more than evil micros or lamp skirt hides. Using your imagination, they can incorporate humor in the theme, container, execution, experience, etc. This is a contest to challenge people to include humor in a creative way in a Geocache. How to Enter:
Place a funny cache between now and July 1st. The name of the cache MUST start with LOL! so we know it was intended for this contest. For example, if you want to name your cache "Funny Cache" you would name it "LOL! - Funny Cache" with a hyphen after the LOL! Geographic Boundaries:
Your cache must be placed within a 25 mile radius of the coordinates of this event to be included in the contest. Prize Categories:
Funny Container and Use of Humor in a theme or cache experience. All who are present can vote by secret ballet on the caches as long as they have found more than 60% of the published caches. (A secret ballet is when you have to dance to show which ones you like). Just kidding. It will be a secret ballot.
You do not have to be present to win, but it will be more fun if you are! We'll all be winners if a whole bunch of really fun caches get placed!
Saturday, July 12, 11:00 am Central Market Patio/Picnic area at the published coordinates Everyone is welcome whether they hid a cache or not!
This article appeared on the Geocaching.com Blog!
Get ready for a classic win-win situation from the geocaching playbook: lend a hand to improve the geocaching game board (a local park, trail, or geocaching-friendly location) and earn the 2014 CITO souvenir. It’s easy and rewarding.
It all happens during the 2014 CITO Weekend on April 26 and 27. Everyone who logs an “Attended” at a Cache In Trash Out (CITO) Event earns this year’s CITO souvenir for their Geocaching profile. So start planning your event now!
During a CITO Event, geocachers search parks, trails and forests to earn a smiley—but they’re not looking for geocaches. They’re clearing litter and trash from geocaching-friendly areas around the world. On CITO weekend, thousands of geocachers will walk away from their events with bags of trash and a sense of pride. They’re preserving the natural beauty of our world. It’s often more than litter clean up. Some CITO events remove invasive species, plant trees or build trails.
Join the geocaching community movement. CITO events are held all year long, but you’ll have plenty of events to choose from on April 26 and 27. Last year, over 13,000 geocachers from more than 30 countries combined in the annual worldwide environmental effort, which is tied closely with Earth Day.
Share this CITO video, create a CITO event near you and gear up with CITO trash bags, trackables and more at Shop Geocaching. See you in April for the 2014 CITO Weekend!
Central Texas Geocachers are cleaning up their portion of the highway on April 27th, 2014 @ 1pm.
The following article appeared on the Geocaching.com Blog.
Maker Madness Souvenir Image
Whether you’re a Social Butterfly or a Nature Lover, a Stats Geek, Traveler, or Puzzler (take the quiz and find out your geocacher type), it’s time to say thank you to a geocache maker near you. From March 28 – April 7, geocachers around the world will join together to celebrate the people in their community who make geocaching possible. We call this celebration Maker Madness.
Every geocacher who logs an “Attended” at a registered Maker Madness event will earn a Maker Madness souvenir.
Take a look at these tips for hosting an event that will have your guests inspired to go forth and create their own magical geocaching experiences:
1.) Think ahead. The best geocaches are not just thrown together willy-nilly. They are a labor of love that take time and careful planning. The same should apply to your Geocaching Event. Do you need to make reservations for a venue? Do your guests need to bring their own creative supplies? Keep in mind that you have to submit your Geocaching Event at least 14 days prior to your Maker Madness celebration, which means that Monday, March 24th is the cut off date for your online submission. You will also want to submit the Maker Madness registration form as soon as your event is published.
2.) Think collaboration. Geocaches are a gift to the community. It’s time to get the community together to give back to those who make geocaching tick. To maximize the number of geocachers at your event, check to see if there is already another event published in your area. If there’s already an event, be a team player! See if you can help the event host with planning their event.
3.) Think education and inspiration. Are you an accomplished geocache maker? This is your chance to share your mad skills and big brain with your fellow geocachers. Incorporate a class or workshop into your event to show others how you implemented your best creations, or teach them the very basics of geocache making. Imagine what it would be like if every one of your guests walked out of your event inspired to make their own awesome creation. Imagine all of the cool, new geocaches to find!
4.) Think celebration. We heart geocache makers and we know that their local (and world) communities do too. If you yourself are not a maker, this is your chance to celebrate your favorites! Organize an event to show your appreciation, showcase their geocaches, and maybe even ask them to share a few of their best tips and tricks with all of their fans who attend the event.
We hope that these tips will help you to spread your creative geocaching juices, inspire a new generation of awesome hiders, honor your favorite makers, and (let’s hold nothing back) make your event the very best Maker Madness bash ever.
A few things to remember:
- Dates: March 28 – April 7
- Don’t forget to register your event after it has been published by a reviewer. Events that are not registered will not qualify for a souvenir.
- You can check to see if your event or an event you are attending has been officially registered here.
- Share photos and stories from your event with us on Facebook, Twitter, and/or Instagram.
- And don’t worry – special Maker Madness GeoCoins, Tags, Trackable Patches, and T-shirts coming soon to Shop Geocaching!
From the Geocaching.com blog:
Geocaching is heading towards the International Space Station (ISS). All of geocaching? No, not all of it, but a very special little Travel Bug® named TB5JJN1 is tagging along with Nasa astronaut Rick Mastracchio on his mission to the ISS. Mastracchio will be using the Travel Bug as an educational tool to help teach students around the world about geography and geocaching.
Mastraccio will Rocket into Space on November 7, 2013 (GMT). Every geocacher who attends a Geocaching Event on November 6 or 7, 2013 will be awarded a “Geocaching in Space” souvenir. In additional, Geocaching in Space Mission Patches will be sold, and proceeds will to the educational charity DonorsChoose.org.
Recently Rand McNally posted a blog and a website after taking nominations for "Best of the Road". The categories were for America’s Most Beautiful, Most Fun, Friendliest, Most Patriotic, Best for Food, and (new this year!) Best for Geocaching small towns.
Helena, Montana won the prestige award for the best town to go Geocaching in.
Rand McNally wrote:
"Geocaches on the Helena GeoTour are so rewarding that even locals swear they learn new things about their town. As one poster put it: 'People have reported that [geocaching here] is fun, hard [with] different terrains.' What else would you expect from a Rocky Mountain town in Big Sky Country?"
“I was convinced by the words taken directly from the geocaching community: this is an authentically engaging place to geocache. You can feel the historical connection to the gold treasure hunters from long ago while using today's satellites and smartphone apps. The gorgeous scenery just adds to the adventure!”
Check out more at the "Best of the Road" website
. They have the runners up listed and photographs and comments from readers on their site.
|by Keith (Indigo Parrish)
|| Picture from the Event has been added!
The friends of River Place Trail want to hold a rally at Sun Tree park
at 7pm on Oct 12, 2013 to show public support for reopening the River
Place Nature Trail segments that have been closed to hikers (and
Note: The trails to the 'Solar System' and 'Rainbow'
cache series are the ones that were affected. It would be great to have
those trails re-opened and the geocaching community certainly has a
vested interest in showing support to do that.
So why not host a geocaching Event at the rally? That should help with turnout.
Are there any River Place cachers willing to host an event at 7pm Oct 12 there?
Check out the event that is happening to support the trails:
Sunset Rainbow Flash Mob and Travel Bug party (GC4P2TC
Red, Orange, Yellow, Green, Blue, Indigo, Violet
N 30° 22.660 W 097° 51.200
Please arrive by 6:55pm and randomly mill about the Soccer field and Park area
Then at sunset (7:03pm) we'll all line up toward the center of the
Soccer Field in Rainbow Order L-R facing West for a group photo: Red, Orange, Yellow, Green, Blue, Indigo, Violet
Again please arrive by 6:55pm and randomly mill about the Soccer field and Park area
We want a COLORFUL PHOTO so please wear a bright T-shirt of a color of the rainbow. you got it
Red, Orange, Yellow, Green, Blue, Indigo, or Violet or maybe several of those colors!
Then at sunset (7:03pm) we'll all line up toward the center of the
Soccer Field in Rainbow Order L-R facing West for a group photo:Red, Orange, Yellow, Green, Blue, Indigo, Violet
After the group photo we can trade bugs and stories at the nearby benches and pavilion
Click the image to enlarge!