

Compensate for Google Maps inaccuracy - reion
https://coderwall.com/p/18e3ug

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lutusp
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....](https://maps.google.com/maps?q=hangzhou&hl=en&ll=30.334602,120.140111&spn=0.012594,0.019226&sll=47.272986,-120.882277&sspn=5.069316,9.84375&hnear=Hangzhou,+Zhejiang,+China&t=h&z=16)

... 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."

