Question: Assume that my current point is lat = 50.000 and long = 50.000 and I want to show some bus stations on these location but my limit should depends on zoom level. 

So far, for that aim I find a way: If a can take left-up corner and right-down corners lat's long's ; I will find stations between these locations.

Do you know how can I take these points or any different idea about this situation?

Thanks in advance..

Answer: 
Well you can easily get the top left and bottom right lat/lon coordinates using

```
GeoPoint tlGpt; // Top left
GeoPoint brGpt; // Bottom right

tlGpt = mapView.getProjection().fromPixels(0, 0);
brGpt = mapView.getProjection().fromPixels(mapView.getWidth(), mapView.getHeight());

```

At any zoom level where you could actually ***see*** the bus stations on the map, then the top left longitude could be considered the same as the bottom left longitude, similarly bottom left long would be approx = bottom right long. Thus you could consider the bounding box as a rectangle, rather than an isosceles trapezium (trapezoid in US English)
If you just want a rough measure to pick up a reasonable sample of objects, then lat/long "distance" is good enough. Just make sure you divide the latitude range by the cosine of the latitude, to get the longitude range (i.e., `∆long = ∆lat/cos(lat)`). This is to compensate for the contraction of longitude lines as you approach the poles. You use ∆lat as the basis because latitude lines have the same distance between them everywhere on the globe.

For a more accurate measure, there are some complicated functions that allow you to compute great circle distances from lat/long pairs, but it is conceptually much easier to convert lat/long pairs into 3-D coordinates, and use a simple pythagorean distance to approximate the great circle distance. You could use `2*r*acos(d/(2*r))` (if my whiteboard geometry serves me well), where r is the nominal radius of the earth, to get the exact great circle. But if all you want is to get objects within a range, you can invert the formula to get the pythagorean-distance equivalent of the great-circle limit. This can also be used to derive a 3-D bounding box to speed up the search. If your database supports R-trees, then you're laughing! SQLite supports R\*Trees, but they are disabled in the default build, so I don't know if they're available on Android (it seems that [it isn't](https://stackoverflow.com/questions/4285462/android-sqlite-with-rtree-module)).