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170

Accuracy is the tendency of your measurements to agree with the true values. Precision is the degree to which your measurements pin down an actual value. The question is about an interplay of accuracy and precision. As a general principle, you don't need much more precision in recording your measurements than there is accuracy built into them. Using too ...


55

The Wikipedia page Decimal Degrees has a table on Degree Precision vs. Length. Also the accuracy of your coordinates depends on the instrument used to collect the coordinates - A-GPS used in cell phones, DGPS etc. decimal places degrees distance ------- ------- -------- 0 1 111  km 1 0.1 11.1 km ...


45

In short, the distance can be in error up to roughly 22km or 0.3%, depending on the points in question. That is: The error can be expressed in several natural, useful ways, such as (i) (residual) error, equal to the difference between the two calculated distances (in kilometers), and (ii) relative error, equal to the difference divided by the "correct" ...


17

There are a lot of factors influencing GPS accuracy. James Ryan is right about night time and clear atmospheric conditions. Then there is the satellite "layout" on a specific time at at specific position on earth. Depending on how many you can "see" (4 at least for a 3D fix) and their distribution (all in one row is bad, evenly spread out better) your ...


16

The "accuracy" figure displayed by your GPS receiver will most likely be fairly reliable, but it can also be 'way off'. It isn't "accuracy" that the GPS is displaying - it is the EPE, which is the Estimated Position Error. In other words, it is the probability that the location the GPS is displaying is within the "accuracy" distance from the true location. ...


15

There are a huge number of devices to choose from, and it is difficult to broadly say that one family of device are more precise than others. It is better to compare accuracy based on real life tests. However, there are a few things to consider: Chipsets. Dedicated GPS, and Smart Phones use a chipset to actually perform the GPS calculation. The ...


14

I saw a study at New Mexico State comparing an HTC G1 and a Trimble Juno. You might want to check it out. Here are the study's accuracy test results:


13

Great Question: Can Time and Position affect your GPS accuracy. Short Answer: YES Why? First lets place your GPS Receiver in perfect conditions where the atmospheric conditions are perfect, there are no multipath effects, no radio interference, and line of sight to GPS Satellites is clear. Your Time and Position on Earth will determine where the GPS ...


12

I will be purchasing ArcGIS Server from ESRI. The sales people assure me that it will solve all these problems.


12

This illustration stuck with me, and helps me remember at the most basic level what precision vs. accuracy is.This is the source of the image, also containing a little more context. In general, Precision is the how close your grouping of measurements are. Accuracy is how close your measurement is to the actual measurement in the real world. Blah238 is ...


11

As I keep all my data projected in moon units, the error is insignificant.


10

No, there is no absolute value for RMS, because it depends on the quality of the map being georeferenced, the quality of the target (base) map, and the purpose of the georeferencing. In particular, any advice that relates RMS to cellsize is misinformed, because cellsize reflects precision in the digital representation of an image whereas the RMS error ...


10

The United States government currently claims 4 meter RMS (7.8 meter 95% Confidence Interval) horizontal accuracy for civilian (SPS) GPS. Vertical accuracy is worse. Mind you, that's the minimum. Some devices/locations reliably (95% of the time or better) can get 3 meter accuracy. For a technical document on that specification you can go here. For more ...


9

In ArcGIS 10, check out IGeometryServer2 which now has GetDistanceGeodesic (geodesic distance between two geometries), GetLengthsGeodesic (return the geodesic lengths of each polyline), and DensifyGeodesic (densify a polyline by plotting points along the geodesic lines connecting the vertices, uses IPolycurve4::GeodesicDensify) methods. As mentioned in the ...


9

If you want a stable method of computing geodesic distances, I recommend Richie Carmichael's wrapper for ESRI's Projection Engine. Update: I just tried Richie's code with ArcGIS 10.0 on Vista64 and get an exception after calling LoadLibrary. I'll look into that more later. For now though, here is some code in response to questions in the comments of ...


9

With qualifications and suitably modified, this is correct: the standard deviation measures spread, which is inversely related to precision, while the vectors (not distances) to the reference points measure inaccuracy. Discussion With only five readings per location several problems will arise: The standard deviations of the coordinates will vary--by a ...


8

Apparently, the measurment ruler is not accurate over long distances > 5,000 km. Google's offical stance says, "makes no claims as to the accuracy of the coordinates in Google Earth. These are provided for entertainment only and should not be used for any navigational or other purpose requiring any accuracy whatsoever".


8

The best accuracy you are going to get with a phone or tablet device is 20-30 meters. So you have to ask yourself what is the purpose of your GPS? If you intend on navigating back to the asset in question, do you really want your users within 20-30 meters, is that accurate enough for what you are attempting to capture. What the higher end GPS receivers give ...


8

I've explored this question recently. I think people want to know what spherical radius should I use? what is the resulting error? A reasonable metric for the quality of the approximation is the maximum absolute relative error in the great-circle distance err = |s_sphere - s_ellipsoid| / s_ellipsoid with the maximum evaluated over all possible pairs ...


8

Ionospheric delay effects are the largest source of error in a single-frequency GPS receiver. WAAS and CORS are able to correct for this better than a receiver's almanac, so the best you can do with uncorrected GPS is typically about 15 meters. Survey-grade GPS using RTK is able to achieve centimeter accuracy. Image source: ...


7

The accuracy of any answer about ArcGIS is subject to change at any time--for all we know, new procedures will be introduced in the very next service pack without warning or documentation. That being said, ESRI software has for a long time used Euclidean calculations (e.g., the Pythagorean formula for distances) whenever projected coordinates are used. ...


7

According to the Exif standards, yes there are attributes relating to GPS accuracy such as GPSDOP (Dilution of precision). See this table for a list of these tags. You could also use a tool like ExifTool to extract and view the Exif data yourself. However, not all devices actually use GPS but use other forms of geolocation such as cellular ...


7

I just happened across another published study on this. Paul Zandbergen, 2009, Accuracy of iPhone Locations: a Comparison of Assisted GPS, WiFi, and Cellular Positioning. Transactions in GIS, 13(s1): 5-26. (PDF, 1mb) They found median errors of 8 m with GPS-assisted 3G iPhones, down to median 47 m errors for WiFi, and median errors of 600 m (!) for ...


6

Here's my rule of thumb table... Latitude coordinate precision by the actual cartographic scale they purport: Decimal Places Aprox. Distance Say What? 1 10 kilometers 6.2 miles 2 1 kilometer 0.62 miles 3 100 meters About 328 feet 4 10 meters About 33 feet 5 ...


6

I'll try to explain it in different terms: Earth's equatorial circumference is about 40,000 kilometers (25,000 miles). A latitude/longitude value breaks that distance up into 360 degrees, starting at -180 and ending at 180. This means that one degree is 40,000 km (or 25,000 miles) divided by 360: 40,000 / 360 = 111 25,000 / 360 = 69 (So, one degree ...


6

We have noted that with even some of the more noteworthy (commercial quality not retail) tablet makers, the GPS is not strong enough for central regions of Australia or when there is any kind of interference. Whereas say GETACS maintain a connection even when indoors. Cold boot times can also be in the double digit minutes for the tablets too. This may be ...


6

Many cell phones and tablets will also use Wi-Fi data to assist their location fix. Google has a huge database of wireless access points and their locations, when the phone's Wifi is turned on it will check nearby wireless APs and use that to narrow down your location quickly. It's the same idea as using a cell tower to locate you, but a Wifi network's range ...


6

Averaging only makes sense if you assume that the "noise" in your location measurements is roughly symmetrical - evenly distributed in every direction. That is, for any one measurement, it's equally likely to be wrong in any particular direction. It is probably possible that you could get a noise distribution that isn't symmetrical. For example, if your GPS ...


5

I am getting going with doForms which is built on the ODK system for android. For Natural Resource Applications there is just the ticket. Previously I have run QGIS on the a 13" laptop with a gps plugged in, but doForms is miles ahead of this, because you are trying to catpure data - not make maps. So my suggesting is the hardware is more than appropriate, ...


5

Some students at Skidmore College recently did a simple experiment and found the differences were several meters apart from one another, but that's all. http://academics.skidmore.edu/blogs/onlocation/2012/03/30/smartphone_accuracy/



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