Lat Long

This has been a fun thread to watch and lots of interesting facts about GPS systems and how they work. But none of that is really relavent to my original post/question. Which is in a nut shell.
The lat long for a RV Park is listed as: ( for instance )
34.3578 -112.6532.
And when I enter that information into say GE it plots a point on the map that is not a RV Park.
After a little searching, usually by entering the name, city State of said RV park I find that it is in fact at: ( again for example )
34.5722 -112.6355

So what is going on is the information is incorrect. I was mostly wondering what the source was for these numbers. I find this is the case ( as an estimate ) about 25% of the time. Thus my 25% number.

That's all. Thanks all for the very intertaining and informative discussion.

OK, let me put my geek hat on for a bit. The coordinates in the GS book may or may not be GPS derived. Remember, GPS has to do with the satellites and the receiver. I would guess many of the "bad" coordinates are from a surveyor who generated them maybe 30 years ago. With modern GPS, if you go park your GPS receiver at the office/gate/whatever, the latitude and longitude it gives you is going to pretty accurate (easily 10's of feet). Datum that SCVJeff talked about were used by surveyors and some early aerial photographic techniques and could account for some of the error. There is also the old, "I copied it wrong" error and the "I am a human and made a mistake in my survey" error.

To hopefully clear up a few things (maybe cause more confusion, I don't know): The earth is closer to a geoid than an ellipsoid but GPS uses an ellipsoid because it is easy to describe mathematically and it has a unique correspondence to the Earth-Centered-Earth-Fixed (ECEF) coordinate system the GPS uses for navigation.

GPS coordinates are based on the WGS-84 ellipsoid. None of my GPS units allows me to select a datum and nothings I have seen leads me to believe these GPS translate the latitude and longitude into any other system. Joe surveyor (or campground owner) may use his GPS to determine a location then have the GPS translate them into a different datum (this is additional software), but I do not see anything that indicates RV/car/Truck GPS systems do this translation.

GPS assumes the earth is an ellipsoid then uses a geoid model to determine the Mean Sea Level for a given location and an Earth Gravity Model (EGM) to generate the terrain elevation. The terrain elevation (where the dirt ends and the air begins) was generally derived by Shuttle radar measurements but may come from other sources too. All the none "average elevation" stuff is easily accounted for.

Virtually all GPS receivers use the same ellipsoid to determine latitude and longitude. I do not know of any GPS that does not use WGS-84. In addition, with modern GPS, two receivers should give virtually the same location even if they are not tracking the same satellites.

GPS accuracy has some dependence on your location on the earth, more correctly your location relative to the GPS satellites but for most of our travels, that is negligible.

A note about precision and accuracy, do not confuse the two. In this application, precision is more or less how many numbers are to the right of the decimal point. For instance, 1.001 is less precise than 1.00100000000. The one with all the extra zeros is more precise. Accuracy is how close you are to the correct answer. If the true answer is 1, then they are both inaccurate by 0.001, all the extra zeros just means you are more precise in your error. That is the difference in a nut shell. As far as the relationship between precision and accuracy, there is obviously a connection that if you do not have enough precision, your accuracy will suffer. In other words, if I put things in whole number miles, I will have a hard time getting anything accurate to within feet. In the world of latitude and longitude, the number or fraction of feet represented by the digits in a number changes over the earth. In my home town near St Louis, MO, a foot north-south and east-west is represented by approximately 0.000003 degrees. In other words, the position 38.842266, -90.315859 is about 1 foot north and 1 foot east of the position 38.842263, -90.315856. So if you add more digits to the right of the decimal point (e.g 38.842266454654541), you are adding inches/fractions of inches to the position which, in our case, has no value in accuracy.

Since I am retired I no longer have a proof reader so I apologies for my math, format, grammar, typo and spel'n errors.

westernrvparkowner wrote:

RV App Developer wrote:

2gypsies wrote:
From another post by the OP, he is creating an app from the Good Sam Directory. Legal?

All I am using from the Directory is information that is available from all over the internet. I bought the Goodsam directory in an effort to get the most accurate, complete information available. So yes what I am doing is legal.

It sure sounds like you are using vast amounts of data from the Good Sam Guidebook. You are using it for your co-ordinates. In another post you were using it to verify restrooms and showers. Copyright law is pretty clear that simply re-arranging information from a singular, or even multiple, sources is illegal. You cannot, for example, write a book about a Great White Shark you call Valdormountain, terrorizing a small resort town and have the town saved by a young warlock named Harrison Pottery and a boat captain named Quad. Peter Benchley and J.K. Rowling would have issues.
If the Television show "The Profit" is any reflection on Good Sam's CEO, Marcus Lemonis, business philosophy, you can be assured he will vigorously defend the Guidebook's copyrights. And I am reasonably sure he has pockets deep enough to do so rather effectively. Heck, CNBC might even think it would make for good television and provide legal and financial support in exchange for watching Good Sam attempt to crush a copyright infringer.

None of the information I am using is copyrighted in fact it is all public information. Information that is also availabe from other sources such as the RV park's own web site. Just pick any RV park in the Goodsam directory and google the name to see what I mean. Or Just pick a town and google RV park near that town. I know this because I am using those public sources of information as well. The Goodsam directory is only one tool in my tool chest.

You mention restrooms & showers, I have to look elsewhere for that data because it's not in the directory. GPS data is not accurate so I have to look elsewhere for that data. Just to name a few. My database will be more complete & more accurate than any single source I am using.

I am providing free advertising for Goodsam and the RV parks in question. My app is just one of many ways a potential customer can find a goodsam or any other RV park including other smart phone apps. Most businesses are grateful for every potential customer that gets information about their business as long as it's accurate. But just to be sure I contacted Goodsam & Woodalls well in advance to see if they had any objections they did not, their only request was that I insure the information I provide be accurate & up to date. Which is why all information is double checked and verified using multipule public sources of information. Nice try but no cigar.......:)

4X4Dodger wrote:
This is a bit of a complex subject.

First: the earth is NOT round it is an ellipsoid.

Second: All Sat GPS coordinates are based on a "Datum" that can date back as far as the early 1920's or even the 1700's or as late as the 1980's There are many datums that can be used.

Also the earth is not "flat" ie. level all around it's surface. It's mountainous, has valleys etc. For GPS to work it must assume that the earth has an average elevation.

This means that for the purposes of "finding" your exact position and translating that to the ancient system of Latitudes and Longitudes means there will be differences from one gps system to the next depending on the "Datum" used.

If my memory serves, Nigel Calders book "How to Read a Nautical Chart" has a thorough explanation of how this all works and it's worth reading for anyone with a gps.

Therefore one GPS will give different sets of coordinates for a given place on the earth than another. Secondly the accuracy of the coordinate system is based upon which part of the earth you are on and how and when the land surveys were done.

My best advice: get and use a paper map. It dispenses with the coordinate system which is unnecessary on land. (for the most part)

You're clearly smarter than the average user, so you should know that we are not talking about a 1700's Datum. The entire US operates under NAD 83, with the exception of one division of the FCC that still, for some stupid reason referrences NAD 27. Either way, in this country, the difference between any applicable Datum loaded in any GPS is feet, nothing more. As far as comparing GPS's, it makes no difference if the world is round, flat, ellipse, or square, if the GPS devices are looking at the same satellites, the results will be identical. They are nothing more than fancy calculators measuring distance and time at 186,000 mi/sec. Translating that identical positioning back to a map is the core problem, and no varience in Datum will result in the errors the OP is seeing.

itself

cbshoestring wrote:
GPS not accurate. How is this news?

Come down my street and Garmin will say "now arriving at destination on left". I live on the right. :S

Go to my brother-in-laws...last thing Garmin says" You are now 5/10ths of a mile from your location". It knows where I am going, but can't provide directions there? Gets me a 1/2 mile away, knows I am a 1/2 mile away....just gives up????????????

A 25% error rate seems a bit high. My experience has been that GPS gets me there 95% of the time....gets me close 3%.....has no idea where I am going 2% of the time :h

Then again, I tend to imput addresses not coordinances. If you can't give me an address, I probably do not want to visit you.

The positional accuracy of a geocoded address depends on the base data. Google Maps does rooftop geocoding. When I geocode my home address in GM, it places the marker dead atop my house which sits 300 feet off the road.

However, many geocoding apps build their base lookup data from U.S. Census Bureau TIGER/Line shapefiles. These files include most of the street segments in the U.S. and have the left/right side beginning/ending house number ranges for each segment. Intermediate addresses are linearly prorated using those beginning/ending house number ranges which are not perfect. And addresses are not necessarily distributed uniformly along a street segment, so the location is going to be a bit off in many cases.

The concept of address geocoding is pretty simple, but the implementation is more art than science at times. (I wrote and support the geocoder for a large software company.)

RV App Developer wrote:

2gypsies wrote:
From another post by the OP, he is creating an app from the Good Sam Directory. Legal?

All I am using from the Directory is information that is available from all over the internet. I bought the Goodsam directory in an effort to get the most accurate, complete information available. So yes what I am doing is legal.

It sure sounds like you are using vast amounts of data from the Good Sam Guidebook. You are using it for your co-ordinates. In another post you were using it to verify restrooms and showers. Copyright law is pretty clear that simply re-arranging information from a singular, or even multiple, sources is illegal. You cannot, for example, write a book about a Great White Shark you call Valdormountain, terrorizing a small resort town and have the town saved by a young warlock named Harrison Pottery and a boat captain named Quad. Peter Benchley and J.K. Rowling would have issues.
If the Television show "The Profit" is any reflection on Good Sam's CEO, Marcus Lemonis, business philosophy, you can be assured he will vigorously defend the Guidebook's copyrights. And I am reasonably sure he has pockets deep enough to do so rather effectively. Heck, CNBC might even think it would make for good television and provide legal and financial support in exchange for watching Good Sam attempt to crush a copyright infringer.

RV App Developer wrote:

2gypsies wrote:
From another post by the OP, he is creating an app from the Good Sam Directory. Legal?

All I am using from the Directory is information that is available from all over the internet. I bought the Goodsam directory in an effort to get the most accurate, complete information available. So yes what I am doing is legal.

Check copyright law... you are stealing content!

2gypsies wrote:
From another post by the OP, he is creating an app from the Good Sam Directory. Legal?

All I am using from the Directory is information that is available from all over the internet. I bought the Goodsam directory in an effort to get the most accurate, complete information available. So yes what I am doing is legal.

Okay let's make this complex subject a lot more simple.

My app, via the iPhone's GPS/Maps system can show places nearby. That is done by providing it, from my database, with DD coordinates. I have been getting those from many sources with varying degrees of accuracy. So what I am actually doing is double-checking the location with Google Maps and correcting my database, which I own, so that my app will be very close to 99% accurate, accounting for my own input errors. I am aware that GPS systems are inaccurate that's because they rely on Geolocation which is it's self inaccurate.

I have tried to send updates to Goodsam but their feedback e mail is broken. So my efforts are centered around making my app and it's database as complete and accurate as humanly possible.

Actually I did did get an answer to my question, in theory, that the coordinates in the book were provided by users over the years and not verified by the publishers of the guide. Thanks all for your feedback.