GAE搜索API中地理点的距离（）函数精度

Question

Does anyone have any experience or know of any documentation regarding the accuracy of the distance() function for geopoints in GAE search API?

I have 2 lat / long geo points, 47 meters apart. The distance is a fact as I have measured it. Google maps also confirms that to this accuracy.

However when I use the distance() function in a search API context, with first point being the origin, the second point is already retuned by the search at 26 meters (< 26 meters). I would expect < 47 or < 48.

Maybe the short distance is an issue. However, I would expect 1, 2 maybe 5 meters error margin, not 20. If it is the case, then not very practical for "query all restaurants within a radius of 50 or 100 meters"

Answer 1

A few things to investigate:

• how accurate are the coordinates you are passing to distance()?
• what is generating the points you are passing to distance()?
• are you truncating the points to some lower precision?

One arc-second of latitude is about 31 meters, so if you want ± 5 meters accuracy, because errors accumulate, then each input needs to be ± 3.5 meters of its actual location, or ± 0.1 arc-seconds of accuracy. In decimal, this is ± 0.00003 degrees.

If you're using GPS to supply the coordinates, this is within its capability, but not reliably. It can guarantee ± 4 meters accuracy 95% of the time, so the best you might expect is ± 6 meters accuracy for distance.

Answer 2

Are you doing your testing in the development environment? Supposedly the distance() function isn't available in the development environment: https://developers.google.com/appengine/docs/python/search/devserver

Maybe its available, but not reliable.