I am using postgres and postgis.
I have Posts which have a geometry, with an attribute visible_within_m which is how many meters from that point the Post should be shown in results.
I can find Posts within some random radius of some random points by doing ST_DWithin(geometry, ST_SetSRID(ST_Point(a, b), 4326), 10000)
However, I want to know how many Posts are visible with a radius of some random point.
How can I look up how many Posts are visible within a radius of some arbitrary point?
Is there a better way to do this?
You can calculate the distance between each point and the center of your circle. If the distance is grater than the radius then it is outside otherwise it's inside.
const EARTH_RADIUS = 6371000;
const toRad = function(num){return num*Math.PI/180};
var calculateDistance =
function(lat1, lon1, lat2, lon2){
var dLat = toRad(lat2 - lat1);
var dLon = toRad(lon2 - lon1);
var a = Math.sin(dLat / 2) * Math.sin(dLat / 2) + Math.cos(toRad(lat1)) *
Math.cos(toRad(lat2)) * Math.sin(dLon / 2) * Math.sin(dLon / 2);
var distance = EARTH_RADIUS * 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
return distance;
}
Instead of using a constant value for the distance, use the value stored in visible_within_m
SELECT * FROM mytable
WHERE ST_DWithin(geometry, ST_SetSRID(ST_Point(a, b), 4326), visible_within_m);
On a side note, st_dwithin with geometries uses the distance unit of the projection, so for 4326 it is a (meaningless) distance in degrees, not in meters.
Related
Athena only allows to calculate the distance of the buffer in decimal degrees but this value varies with respect to the latitude in the globe, tate to obtain a distance according to the following formula but it is not consistent in Mexico.
Athena function like this : ST_Buffer(geometry, double)
Athena geospatial functions
So, is posible obtain the corresponding distance in decimal degrees over a custom point in map , ex : get the decimal degree for point x, y like that distance in meters is 300 mts
Currently I use the following formula to approximate the decimal degrees but some buffers are quite horrible although it meets the minimum required
SELECT
ST_Buffer(ST_GeometryFromText( shape_wkt) ,
abs(5000.0 * 360.0 / (2.0 * pi() * cos( latitud )* 6400000.0) ) ) AS
dinamic_buffer_5000
5000 is buffer in meters
6400000.0 earth radius in meters
Some useffull questions :
gps-coordinates-in-degrees-to-calculate-distances
Calculate distance in meters using results in degrees
calculating-latitude-longitude-x-miles-from-point
A possible alternative is the following
To obtain the decimal degrees relative to a point one could:
Generate a second point at a distance d for this you would have to implement this formula, where the bearing does not matter
With this second point calculate the distance in Athena that will return the distance in decimal degrees, as input for the buffer function.
As an approximate is good alternative
Now how implement the second point ?....Here is the formula
I will try to convert to SQL code if can :
After a test I realize that even with the difference of distance it is not possible to obtain the buffer in an optimal way.
In this case the distance to the lower point was 300 meters, after obtaining the distance in decimal degrees with Athena an oblate shape is obtained, it changes the degree of inclination of the point by 90 degrees but it only generates a slightly larger shape.
Destination point given distance and bearing from start point
Source code (zory im edit for test my sql ):
destinationPoint(distance, bearing, radius=6371e3) {
// sinφ2 = sinφ1⋅cosδ + cosφ1⋅sinδ⋅cosθ
// tanΔλ = sinθ⋅sinδ⋅cosφ1 / cosδ−sinφ1⋅sinφ2
// see mathforum.org/library/drmath/view/52049.html for derivation
const dist_ang = distance / radius; // angular distance in radians
const angulo = Number(bearing).toRadians();
const rad_lat = this.lat.toRadians();
const rad_lon = this.lon.toRadians();
console.log("distance", distance);
console.log("radius", radius);
console.log("angular distance in radians", dist_ang);
console.log("bearing", Number(bearing));
console.log("bearing angulo ", angulo );
console.log("lat.toRadians", rad_lat);
console.log("lon.toRadians", rad_lon);
console.log("lon",this.lon);
console.log("lat",this.lat);
const sinφ2 = Math.sin(rad_lat) * Math.cos(dist_ang) + Math.cos(rad_lat) * Math.sin(dist_ang) * Math.cos(angulo);
const φ2 = Math.asin(sinφ2); //lat
console.log("φ2",φ2); //lat
console.log("sinφ2",sinφ2);
const y = Math.sin(angulo) * Math.sin(dist_ang) * Math.cos(rad_lat);
const x = Math.cos(dist_ang) - Math.sin(rad_lat) * sinφ2;
console.log("y",y);
console.log("x",x);
const λ2 = rad_lon + Math.atan2(y, x); //lon
console.log("λ2",λ2);
const lat = φ2.toDegrees();//lat
const lon = λ2.toDegrees();//lon
console.log("lon2",lon);
console.log("lat2",lat);
return new LatLonSpherical(lat, lon);
}
Basically, I have 2 LatLong values 1 for the user that keeps updating according to his current location and 1 stored on the database. How can I check if these 2 coordinates are in the same city or state.
Note: I used the google maps API https://maps.googleapis.com/maps/api/geocode/json?latlng=33.26944346171845,%2035.20463784635442&sensor=true&key=KEY for both coordinates and compared strings(administrative_area_level_2) to check if the user is in the same city or not. Is there a better way to do it in Flutter?
Thanks in advance!
I think your method is quite reasonable. If you don't want to use API, you can calculate the distance between two coordinates with the below method. Using this, you can assume that those below a certain distance are in the same city or state.
const convertToRadian = x => x * Math.PI / 180;
function getDistanceBetween(lat1, lat2, lng1, lng2) {
const R = 6378137; // Earth’s mean radius in meter
const dLat = convertToRadian(lat2 - lat1);
const dLong = convertToRadian(lng2 - lng1);
const a = Math.sin(dLat / 2) * Math.sin(dLat / 2) + Math.cos(convertToRadian(lat1)) * Math.cos(convertToRadian(lat2)) * Math.sin(dLong / 2) * Math.sin(dLong / 2);
const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
return R * c; // returns the distance in meter
}
I have two locations and I have the formula to get the distance between them but I want to calculate the travel time between them without using google maps or any API calling. Is there any formula to do that in flutter?
I think the real travel time depends on a lot of factors, for example the road you take or the speed limit.
Whitout some complex elaboration, which are usually perfomed by a backend, I don't think it is possible to calculate it.
Anyway, since you have got the distance, you could simply estimate it by giving a supposed speed and then using basic physics formulas.
time = distance / speed.
You cannot find the travel distance between two locations without using the Google API.
However you can get the air distance in meters using this formula in flutter.
double calculateDistance(
double lat1, double lon1, double lat2, double lon2) {
var p = 0.017453292519943295;
var c = cos;
var a = 0.5 -
c((lat2 - lat1) * p) / 2 +
c(lat1 * p) * c(lat2 * p) * (1 - c((lon2 - lon1) * p)) / 2;
var temp = 12742 * asin(sqrt(a));
return (temp * 1000);
}
A quick question about a Lat / Long calculation.
I want to take a value set e.g. Lat: 55.123456 Long -6.123456 and work out the four points that are an arbitrary distance away.
As the given square, I want to work out the value for Latitude on the left and right side. Thus the red lines are 1.5km from the start point. Likewise for the longitude, the blue lines will be 1.5km from the start point. The output will be 4 points, all distances in kilometres.
In short: Latitude + Y = Latitude Value X kilometers away
Working with iPhone at the moment and its for a very rough database calculation.
EDIT: Just to clarify, the distance is so short that curvature (And hence accuracy) is not an issue.
In OBJ-C this should be a decent solution:
float r_earth = 6378 * 1000; //Work in meters for everything
float dy = 3000; //A point 3km away
float dx = 3000; //A point 3km away
float new_latitude = latitude + (dy / r_earth) * (180 / M_PI);
float new_longitude = longitude + (dx / r_earth) * (180 / M_PI) / cos(latitude * 180/M_PI);
Well, for rough calculation with relatively small distances (less than 100km) you may assume that there is 40_000_000/360=111 111 meters per degree of latitude and 111 111*cos(latitude) meters per degree of longitude. This is because a meter was defined as 1/40_000_000 part of the Paris meridian;).
Otherwise you should use great circle distances, as noted in the comments. For high precision you also need to take into account that Earth is slightly oblate spheroid rather than a sphere.
// parameter: offset in meters
float offsetM = 1500; // 1.5km
// degrees / earth circumfence
float degreesPerMeter = 360.0 / 40 000 000;
float toRad = 180 / M_PI;
float latOffsetMeters = offsetM * degreesPerMeter;
float lonOffsetMeters = offsetM * degreesPerMeter * cos (centerLatitude * toRad);
Now simply add +/- latOffsetMeters and +/- lonOffsetMeters to your centerLatitude/ centerLongitude.
Formula is usefull up to hundred kilometers.
As my question states that's I am looking for a function/formula that can calculate a distance between two points. Now I have looked at example and found great functions but none of them seem to work they all return 0 when I supply 2 sets of points. Basically I will need to pass the function the following (lat1,lon1,lat2,lon2) and get back the distance. From this distance I can check a check if another point is close by.
UPDATE
Okay so I am now using the following function,
BEGIN
DECLARE pi, q1, q2, q3 , roundedVal FLOAT ;
DECLARE rads FLOAT DEFAULT 0;
SET pi = PI();
SET lat1 = lat1 * pi / 180;
SET lon1 = lon1 * pi / 180;
SET lat2 = lat2 * pi / 180;
SET lon2 = lon2 * pi / 180;
SET q1 = COS(lon1-lon2);
SET q2 = COS(lat1-lat2);
SET q3 = COS(lat1+lat2);
SET rads = ACOS( 0.5*((1.0+q1)*q2 - (1.0-q1)*q3) );
RETURN FORMAT((6371 * rads) , 1);
END
This works fine with Kilometres, but what I am looking for is meters. So I know I have the change the numbers in that function but which ones and what to. Any help ?
I have used this webiste in the past and it has worked for me. Has lots of useful formulas and gives examples in javascript.
http://www.movable-type.co.uk/scripts/latlong.html
I'd recommend you take a look at a spacial extention to MySQL.
http://dev.mysql.com/doc/refman/5.0/en/spatial-extensions.html
If you don't fancy that, this blog might have some use to you:
http://zcentric.com/2010/03/11/calculate-distance-in-mysql-with-latitude-and-longitude/
Try this query
$qry = "SELECT *,(((acos(sin((".$latitude."*pi()/180)) *
sin((`Latitude`*pi()/180))+cos((".$latitude."*pi()/180)) * cos((`Latitude`*pi()/180))*
cos(((".$longitude."- `Longitude`)*pi()/180))))*180/pi())*60*1.1515) as distance FROM
'MyTable` WHERE distance >= ".$distance."
apply this on the values
double theta = src_longitude - dest_longitude;
double min_distance = (Math.sin(Math.toRadians(src_latitude)) * Math.sin(Math.toRadians(dest_latitude))) +(Math.cos(Math.toRadians(src_latitude)) * Math.cos(Math.toRadians(dest_latitude)) * Math.cos(Math.toRadians(theta)));
min_distance = Math.acos(min_distance);
min_distance = Math.toDegrees(min_distance);
min_distance = min_distance * 60 * 1.1515 * 1.609344;