問題描述

我的 SQLite 數據庫中存儲了經緯度數據，我想獲取與我輸入的參數最近的位置(例如，我當前的位置 - 緯度/經度等).

我知道這在 MySQL 中是可能的，并且我已經進行了相當多的研究，認為 SQLite 需要一個自定義外部函數用于 Haversine 公式(計算球體上的距離)，但我還沒有找到任何寫在Java 和作品.

另外，如果我想添加自定義函數，我需要 org.sqlite .jar(用于 org.sqlite.Function)，這會增加不必要的大小應用程序.

另一方面，我需要 SQL 中的 Order by 函數，因為僅顯示距離并不是什么大問題 - 我已經在自定義 SimpleCursorAdapter 中做到了，但我無法對數據進行排序，因為我的數據庫中沒有距離列.這意味著每次位置更改時都要更新數據庫，這是對電池和性能的浪費.因此，如果有人對使用不在數據庫中的列對光標進行排序有任何想法，我也將不勝感激！

我知道有大量的 Android 應用程序使用此功能，但誰能解釋一下它的魔力.

順便說一句，我找到了這個替代方案:并完成了它.

I have data with latitude and longitude stored in my SQLite database, and I want to get the nearest locations to the parameters I put in (ex. My current location - lat/lng, etc.).

I know that this is possible in MySQL, and I've done quite some research that SQLite needs a custom external function for the Haversine formula (calculating distance on a sphere), but I haven't found anything that is written in Java and works.

Also, if I want to add custom functions, I need the org.sqlite .jar (for org.sqlite.Function), and that adds unnecessary size to the app.

The other side of this is, I need the Order by function from SQL, because displaying the distance alone isn't that much of a problem - I already did it in my custom SimpleCursorAdapter, but I can't sort the data, because I don't have the distance column in my database. That would mean updating the database every time the location changes and that's a waste of battery and performance. So if someone has any idea on sorting the cursor with a column that's not in the database, I'd be grateful too!

I know there are tons of Android apps out there that use this function, but can someone please explain the magic.

By the way, I found this alternative: Query to get records based on Radius in SQLite?

It's suggesting to make 4 new columns for cos and sin values of lat and lng, but is there any other, not so redundant way?

1) At first filter your SQLite data with a good approximation and decrease amount of data that you need to evaluate in your java code. Use the following procedure for this purpose:

To have a deterministic threshold and more accurate filter on data, It is better to calculate 4 locations that are in radius meter of the north, west, east and south of your central point in your java code and then check easily by less than and more than SQL operators (>, <) to determine if your points in database are in that rectangle or not.

The method calculateDerivedPosition(...) calculates those points for you (p1, p2, p3, p4 in picture).

/**
* Calculates the end-point from a given source at a given range (meters)
* and bearing (degrees). This methods uses simple geometry equations to
* calculate the end-point.
* 
* @param point
*           Point of origin
* @param range
*           Range in meters
* @param bearing
*           Bearing in degrees
* @return End-point from the source given the desired range and bearing.
*/
public static PointF calculateDerivedPosition(PointF point,
            double range, double bearing)
    {
        double EarthRadius = 6371000; // m

        double latA = Math.toRadians(point.x);
        double lonA = Math.toRadians(point.y);
        double angularDistance = range / EarthRadius;
        double trueCourse = Math.toRadians(bearing);

        double lat = Math.asin(
                Math.sin(latA) * Math.cos(angularDistance) +
                        Math.cos(latA) * Math.sin(angularDistance)
                        * Math.cos(trueCourse));

        double dlon = Math.atan2(
                Math.sin(trueCourse) * Math.sin(angularDistance)
                        * Math.cos(latA),
                Math.cos(angularDistance) - Math.sin(latA) * Math.sin(lat));

        double lon = ((lonA + dlon + Math.PI) % (Math.PI * 2)) - Math.PI;

        lat = Math.toDegrees(lat);
        lon = Math.toDegrees(lon);

        PointF newPoint = new PointF((float) lat, (float) lon);

        return newPoint;

    }

And now create your query:

PointF center = new PointF(x, y);
final double mult = 1; // mult = 1.1; is more reliable
PointF p1 = calculateDerivedPosition(center, mult * radius, 0);
PointF p2 = calculateDerivedPosition(center, mult * radius, 90);
PointF p3 = calculateDerivedPosition(center, mult * radius, 180);
PointF p4 = calculateDerivedPosition(center, mult * radius, 270);

strWhere =  " WHERE "
        + COL_X + " > " + String.valueOf(p3.x) + " AND "
        + COL_X + " < " + String.valueOf(p1.x) + " AND "
        + COL_Y + " < " + String.valueOf(p2.y) + " AND "
        + COL_Y + " > " + String.valueOf(p4.y);

COL_X is the name of the column in the database that stores latitude values and COL_Y is for longitude.

So you have some data that are near your central point with a good approximation.

2) Now you can loop on these filtered data and determine if they are really near your point (in the circle) or not using the following methods:

public static boolean pointIsInCircle(PointF pointForCheck, PointF center,
            double radius) {
        if (getDistanceBetweenTwoPoints(pointForCheck, center) <= radius)
            return true;
        else
            return false;
    }

public static double getDistanceBetweenTwoPoints(PointF p1, PointF p2) {
        double R = 6371000; // m
        double dLat = Math.toRadians(p2.x - p1.x);
        double dLon = Math.toRadians(p2.y - p1.y);
        double lat1 = Math.toRadians(p1.x);
        double lat2 = Math.toRadians(p2.x);

        double a = Math.sin(dLat / 2) * Math.sin(dLat / 2) + Math.sin(dLon / 2)
                * Math.sin(dLon / 2) * Math.cos(lat1) * Math.cos(lat2);
        double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
        double d = R * c;

        return d;
    }

Enjoy!

I used and customized this reference and completed it.

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