I have a set of points on a map.
For each point (say 'a'), i want to obtain n (say 5) nearest neighbours that surround the point.
Currently, I make a list of euclidean distance from 'a' to other points .Then I sort the list and choose 'n' smallest values .
I found that this way, it sometimes happen that all closest points will form a polygon without containing a.
Can you suggest a way to choose n points that surround a.
I know the method to determine if a point lies in polygon. I am not able to decide which closest point to discard when I include n+1th point of neighbourhood.
Thanks.
Related
I want to know something more about the latDist component in traci.vehicle.changeSublane(vehID, latDist) rather than what sumo says in "https://sumo.dlr.de/docs/TraCI/Change_Vehicle_State.html#lane_change_mode_0xb6". Does it have any interval? Do the values it takes are the matter of distance? Does it have values as threshold? What do we mean when using for instance "3.00" as latDist?
Best,
Ali
In SUMO's sublane model every vehicle has a continuous lateral position meaning it can be freely positioned in the boundaries of the edge, occupying one or more sublanes. This means a "lane change" is nothing more than a lateral movement. To make it independent of the actual sublane width (which has not so much relevance in reality) the offset to change is now given in meters and not in lane (or sublane) numbers. So an offset of 3.0 means move 3 meters to the left (in a right hand driven network).
I have a set of points and I am doing a CGAL::Delaunay_triangulation_2 with them. However, the order of the points in the resulting triangulation is not the same as in the input points. For example, is input point 0 is in (-1,-1), the output point 0 in the triangulation is not in the same position. The point in position (-1,-1) is another one, but not necesarily the 0th.
For me, it is important to keep the order, as I am taking some references (as indices) to the original set of points, so I need the vertex number i in the input set and in the output set to be the same one.
Is there any way to make the output set be ordered the same as the input set? I dont care if I need to reorder the input set, as I can easily do that before taking the references.
As documented here: "Note that this function is not guaranteed to insert the points following the order of PointInputIterator, as spatial_sort() is used to improve efficiency."
If you insert your points one by one then they will be in the insertion order (provided there is no duplicate).
See also this example that can be used to set input id as info() of vertices (then a vector can be created to have direct access from id -> vertex).
I'm looking at the freely available Solar potential dataset on Google BigQuery that may be found here: https://bigquery.cloud.google.com/table/bigquery-public-data:sunroof_solar.solar_potential_by_censustract?pli=1&tab=schema
Each record on the table has the following border definitions:
lat_max - maximum latitude for that region
lat_min - minimum latitude for that region
lng_max - maximum longitude for that region
lng_min - minimum longitude for that region
Now I have a coordinate (lat/lng pair) and I would like to query to see whether or not that coordinate is within the above range. How do I do that with BQ Standard SQL?
I've seen the Geo Functions here: https://cloud.google.com/bigquery/docs/reference/standard-sql/geography_functions
But I'm still not sure how to write this query.
Thanks!
Assuming the points are just latitude and longitude as numbers, why can't you just do a standard numerical comparison?
Note: The first link doesn't work without a google account, so I can't see the data.
But if you want to become spatial, I'd suggest you're going to need to take the border coordinates that you have and turn them into a polygon using one of: ST_MAKEPOLYGON, ST_GEOGFROMGEOJSON, or ST_GEOGFROMTEXT. Then create a point using the coords you wish to test ST_MAKEPOINT.
Now you have two geographies you can compare them both using ST_INTERSECTION or ST_DISJOINT depending on what outcome you want.
If you want to get fancy and see how far aware from the border you are (which I guess means more efficient?) you can use ST_DISTANCE.
Agree with Jonathan, just checking if each of the lat/lon value is within the bounds is simplest way to achieve it (unless there are any issues around antimeridian, but most likely you can just ignore them).
If you do want to use Geography objects for that, you can construct Geography objects for these rectangles, using
ST_MakePolygon(ST_MakeLine(
[ST_GeogPoint(lon_min, lat_min), ST_GeogPoint(lon_max, lat_min),
ST_GeogPoint(lon_max, lat_max), ST_GeogPoint(lon_min, lat_max),
ST_GeogPoint(lon_min, lat_min)]))
And then check if the point is within particular rectangle using
ST_Intersects(ST_GeogPoint(lon, lat), <polygon-above>)
But it will likely be slower and would not provide any benefit for this particular case.
I have an application with a sqlite database that contains 7000+ records in it with city names, longitudes and latitudes.. also these "cities" are connected to relevant city fields on the database too.
What my app doing is, query the current location with core location, fetch the lon and lat values, and then find the closest location from the database.
The result doesn't have to be super accurate (i just want to match cities), so I want to use Hypotenuse formula for finding the closest point:
closest city in db: min((x1-x2)^2 +(y1-y2)^2)^(1/2)
x1, y1: lon and lat for user
x2, y2: lon and lat for points in database.
If I was using ms-sql or sqlite database, I could easily create a query but when it comes to core data, I'm out of ideas.
I don't want to fetch all the data (and fill the memory) then aggregate this formula on all fields so is there a way to create a query and get the result from the db?
Am I overthinking this problem, and missing a simple solution?
If I'm understanding your problem correctly, you're wanting to find the closest "n" cities to your current location.
I had something similar and here's how I approached it.
In essence, you probably need to take each city's lat/lon and hash it into some index. We use a Mercator Projection to convert the lat/lon to x/y, then hash that value in a manner similar to how Google/Bing/Apple Maps hash their map tiles. Fortunately, MapKit has a built-in Mercator Projection function.
In pseudocode:
for each city's lat/lon {
CLLocationCoordinate2D coordinate = (CLLocationCoordinate2D){lat, lon};
MKMapPoint point = MKMapPointForCoordinate(coordinate);
//256 represents the size of a map tile at zoomLevel 20. You can use whatever zoomLevel
//you want here, but we need something to quickly lookup close-by cities.
//this is the formula you can use to determine how granular your index is
//(256 * pow(2, (20 - zoomLevel)))
NSInteger x = point.x/256.0;
NSInteger y = point.y/256.0;
save x & y in a CityHashIndex table
}
Now, you get the current location's lat/lon, hash that into the index as above, and just simply write a query against this CityHashIndex table.
So say that, for simplicity sake, you're current location is indexed at 1000, 1000. So to find close by cities, maybe you search for cities with indexes in the range of `900-1100, 900-1100'.
From there, you're now only pulling in a much smaller set of cities and the memory requirements to process your Hypotenuse Formula isn't so bad.
I can elaborate more if you're interested.
This is directly related to a commonly asked question about Core Data.
Searching for surrounding suburbs based on latitude & longitude using Objective C
Calculate a bounding box around the point you need (min lat/long max lat/long) then use an NSPredicate against those values to find everything within the box. From there you can do a distance calculation on the results that return and sort them.
I would suggest setting this up so that it can search at multiple distances then you can see if a city is within 10 miles, 100 miles, etc. Slowly increasing the bounding box until you get one or more results back.
I would use NSPredicate to define my search criteria it will act as a filter. I'm not sure how optimized is this and if it will pull all your registers but I'm assuming that coreData has some kind of indexing mechanism that will optimize the search.
You can take a look of this document
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreData/Articles/cdFetching.html
Check the section named
Retrieving Specific Objects
I have a requirement to calculate the lat and long values of the current position of the user. However, I can't use GPS/Network. I know a previous lat long location of the user. This previous location has been queried from the GPS provider. After this initial location is found, GPS is no more available. User travels a certain distance from this point and in certain direction. Both these values, distance and direction of travel (in terms of angle), are known. Is there any way that I can arrive at the new lat/long coordinates based on this available information (previous lat/long coordinates, distance & direction traveled from the previous position).
This method of navigation is known as dead reckoning: Given is an initial position, the task is to deduce the current position from known information, e.g. heading, time travelled, and speed (or heading and distance).
You may find some formulas to compote the new location here.
Stefan gives you a good place to start. However, depending on what you want to do with that information, you might be better to use a Kalman Filter, which would allow you to account for error in both the starting position, distance traveled and direction traveled. This is especially true if the user is isn't moving just once, but several times.
This link GeoTools - How to do Dead Reckoning and course calculations using GeoTools classes answers my question. Given an angle, distance and starting geographic point, GeoTools (java) can be used to do the calculation. Refer to the SO link to see the sample.