Take all screen coordinates

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 w开发者_开发问答ill find stations between these locations.

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

Thanks in advance..

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).