Compensate for Google Maps inaccuracy

Compensate for Google Maps inaccuracy(coderwall.com)

1 points by reion 13 years ago | 1 comment

lutusp 13 years ago |

This inaccuracy may not arise from what you think. Each map has a "datum", a coordinate correction that obeys one or another convention. Examples include NAD83, NAD27, GRS80 and WGS84. Which of these corrections are applied may change the location of sites on the resulting map when it's compared to satellite-derived positions. And a GPS receiver must be told which datum to use.

http://en.wikipedia.org/wiki/North_American_Datum

http://geography.about.com/od/geographyintern/a/datums.htm

A quote from the second link: "Today, there are hundreds of different datums in use; but, they are all either horizontal or vertical in their orientation. The horizontal datum is the one that is used in measuring a specific position on the earth's surface in coordinate systems such as latitude and longitude. Because of the different local datums (i.e. those having different reference points) the same position can have many different geographic coordinates so it is important to know which datum the reference is in."

So it may not be as simple as saying "Google maps inaccuracy", even though that's the outcome.

> That is when I realized satellite maps is fine, but road map is off by some distance.

This sounds as though the satellite map used one datum and the road map another. This is fairly common in locales other than the continental U.S. and Europe.

I just performed a comparison for a freeway intersection in Hangzhou:

https://maps.google.com/maps?q=hangzhou&hl=en&ll=30....

... and came up with an approximate error of 521 meters between the satellite and road maps. This is a rather extreme case, but it could easily be explained by the use of one datum for the satellite map and another for the road map. Here's a reference about this:

Source: http://en.wikipedia.org/wiki/North_American_Datum

Quote: "A point having a given latitude and longitude in NAD27 may be displaced on the order of many tens of meters from another point having the identical latitude and longitude in NAD83, so it is important to specify the datum along with the coordinates."