My question is somewhat related to this similar one, which links to a pretty complex solution - but what I want to understand is the result of this:
Using a Mysql Geometry field to store a small polygon I duly ran
select AREA(myPolygon) where id =1
over it, and got an value like 2.345. So can anyone tell me, just what does that number represent seeing as the stored values were long/lat sets describing the polygon?
FYI, the areas I am working on are relatively small (car parks and the like) and the area does not have to be exact - I will not be concerned about the curvature of the earth.
2.345 of what? Thanks, this is bugging me.
The short answer is that the units for your area calculation are basically meaningless ([deg lat diff] * [deg lon diff]). Even though the curvature of the earth wouldn't come into play for the area calculation (since your areas are "small"), it does come into play for the calculation of distance between the lat/lon polygon coordinates.
Since a degree of longitude is different based on the distance from the equator (http://en.wikipedia.org/wiki/Longitude#Degree_length), there really is no direct conversion of your area into m^2 or km^2. It is dependent on the distance north/south of the equator.
If you always have rectangular polygons, you could just store the opposite corner coordinates and calculate area using something like this: PHP Library: Calculate a bounding box for a given lat/lng location
The most "correct" thing to do would be to store your polygons using X-Y (meters) coordinates (perhaps UTM using the WGS-84 ellipsoid), which can be calculated from lat/lon using various libraries like the following for Java: Java, convert lat/lon to UTM. You could then continue to use the MySQL AREA() function.
Related
I have a SQL database set of places to which I am assigned coordinates (lat, long). I would like to ask those points that lie within a radius of 5km from my point inside. I wonder how to construct a query in a way that does not collect unnecessary records?
Since you are talking about small distances of about 5 km and we are probably not in the direct vicinity of the north or south pole we can work with an approximated grid system of longitude and latitude values. Each degree in latidude is equivalent to a distance of km_per_lat=6371km*2*pi/360degrees = 111.195km. The distance between two longitudinal lines that are 1 degree apart depends on the actual latitude:
km_per_long=km_per_lat * cos(lat)
For areas here in North Germany (51 degrees north) this value would be around 69.98km.
So, assuming we are interested in small distances around lat0 and long0 we can safely assume that the translation factors for longitudinal and latitudinal angles will stay the same and we can simply apply the formula
SELECT 111.195*sqrt(power(lat-#lat0,2)
+power(cos(pi()/180*#lat0)*(long-#long0),2)) dist_in_km FROM tbl
Since you want to use the formula in the WHERE clause of your select you could use the following:
SELECT * FROM tbl
WHERE 111.195*sqrt(power(lat-#lat0,2)
+power(cos(pi()/180*#lat0)*(long-#long0),2)) < 5
The select statement will work for latitude and longitude values given in degree (in a decimal notation). Because of that we have to convert the value inside the cos() function to radians by multiplying it with pi()/180.
If you have to work with larger distances (>500km) then it is probably better to apply the appropriate distance formula used in navigation like
cos(delta)=cos(lat0)*cos(lat)*cos(long-long0) + sin(lat0)*sin(lat)
After calculating the actual angle delta by applying acos() you simply multiply that value by the earth's radius R = 6371km = 180/pi()*111.195km and you have your desired distance (see here: Wiki: great circle distance)
Update (reply to comment):
Not sure what you intend to do. If there is only one reference position you want to compare against then you can of course precompile your distance calculation a bit like
SELECT #lat0:=51,#long0:=-9; -- assuming a base position of: 51°N 9°E
SELECT #rad:=PI()/180,#fx:=#rad*6371,#fy:=#fx*cos(#rad*#lat0);
Your distance calculation will then simplify to just
SELECT #dist:=sqrt(power(#fx*(lat-#lat0),2)+power(#fy*(long-#long0),2))
with current positions in lat and long (no more cosine functions necessary). It is up to you whether you want to store all incoming positions in the database first or whether you want to do the calculations somewhere outside in Spring, Java or whatever language you are using. The equations are there and easy to use.
I would go with Euklid. dist=sqrt(power(x1-x2,2)+power(y1-y2,2)) . It works everywhere. Maybe you have to add a conversion to the x/y-coordinates, if degrees can't be translated in km that easy.
Than you can go and select everything you like WHERE x IS BETWEEN (x-5) AND (x+5) AND y IS BETWEEN (y-5) AND (y+5) . Now you can check the results with Euklid.
With an optimisation of the result order, you can get better results at first. Maybe there's a way to take Euklid to SQL, too.
I'm using an L80 GPS module together with my 8-bit processor. GPS module responds with a massage in NMEA format, giving me information about the date, time, latitude, longitude, altitude (if possible), number of satellites etc.
Latitude and longitude information of NMEA are in the form of degrees and minutes (DD°MM.mmm').
I'm able to convert them into only degrees notation (DD.dddddd°).
I have the following problem: Given a particular location (e.g. 48.858125, 2.294398) and a safety radius of, let's say, 50 meters (no more than 300 meters), how to determine weather (a, b) point is within a safety circle or not?
Can you help me figuring out the math hiding behind?
In short, I would like you to help me determine distance in meters between two points on Earth represented in angular coordinate system. Are there any math guru willing to help me?
Note that my point of calculations is my processor.
I know that, having latitudes and longitudes in degrees, my points are represented in an angular coordinate system, not Cartesian (linear) one.I also know that Universal Transferse Mercator (UTM) representation of points on Earth is in Cartesian coordinate system. Is it, maybe, easier to transform degree notation (DD.dddddd°) into UTM notation? I know there are on-line tools that are able to do a conversion. However, I don't know the math.
Thank you very much for your time and effort to help me.
Sincerely,
Bojan
You can simply find distance b/w two points by longitude and latitude.
you can find reference code on this link.
Hope this helps.
Just use the haversine formula to calcualte the distance between two points on earth.
Search for term "haversine" and the name of your programming language.
Is it, maybe, easier to transform to UTM
No for sure not. It is very complex, and it gets extremly complex when the two points are located in different UTM zones.
I'm trying to generate some annotations on a map. Currently, I'm using openstreetmap but the query is general. I have two end points on a map and a corpus of few selected points I would like to highlight.
The two endpoints are given in the form lat, long
<walkSteps>
<distance>9.742464221826811</distance>
<streetName>5th St NW</streetName>
<absoluteDirection>EAST</absoluteDirection>
<stayOn>false</stayOn>
<bogusName>false</bogusName>
<lon>-84.3937361115149</lon>
<lat>33.77692965678444</lat>
<elevation/>
</walkSteps>
<walkSteps>
<distance>508.2608917548245</distance>
<relativeDirection>LEFT</relativeDirection>
<streetName>Fowler St NW</streetName>
<absoluteDirection>NORTH</absoluteDirection>
<stayOn>false</stayOn>
<bogusName>false</bogusName>
<lon>-84.39363494600667</lon>
<lat>33.77692904176358</lat>
<elevation/>
</walkSteps>
My aim is to highlight those points on the map, which are present in the corpus and lie in the line connecting these two points.
How can I go about querying the corpus for the same? Annotating on map given lat, lng is not an issue
Rounding errors will prevent you from directly doing as you want. What you should be doing instead is determining the great-circle path between the two end points and highlighting those members of the corpus which are within a certain distance of the great circle route. This is known as the cross-track distance or cross-track error. Formulas for computing the cross-track distance can be found at one of the standard reference sites for geospatial equations but there are others as well.. The problem then becomes one of searching for points in the corpus which are close enough to the great circle path between the two end points.
I have a Google Map and a server sends a list of objects that have a position with a small radius (100m max). I need to quickly be able to know if a position is colliding with something in the list and draw on the map everything.
I'm thinking I should use a Quadtree (very useful in 2D collisions for games) but my issue is I'm not limited to a screen but to the earth !
Sure, if I have 100 objects it's not a problem but at any time the server can send me new objects that I need to add to the list and so my Quadtree could drastically change or become unbalanced.
What should I do ? Should I still use a Quadtree and modify the entire tree if a new element is added outside of the current boundaries ? Should I set the boundaries to the max latitude longitude (but could have issue with double precision) ? Or does someone knows a better data structure for that type of problem ?
rXp
To avoid issues with double precision, especially at the splitting border of a quad cell, it is advisable to use integer coordinates in the quad tree.
convert double lat/lon to int by multiplying with 1E6, this results in a precision of about 10cm.
You can use a space-filling-curve, for example a z curve.
I would like to find all points that are within N miles of a given area.
E.g. the area is California: Find all points that are within 50 miles of the border of California (not the middle of California).
When using Google Maps the distance is calculated using 'the middle' of the given location, but I need to calculate the distance using the borders of the given location. The location could be any zip code, city or country.
Could that be done by drawing a polygon using California's coordinates on a map and calculate the distance to location B using the points of the polygon?
Is there a more elegant solution to this? Any ideas?
Thanks!
I'm not sure if I understand your requirements completely, but I will give it a try with different interpretations:
1. You want to filter own map points:
This can be done with any GIS or a own service that offers a call like my_points_in_area(bbox). Bbox means here boundingbox and is the 2x lat/lon pair describing the rectangle around your given centerpoint. If you want to be accurate and really just deliver whats within 100km, you might need to test the distance to the POIs once more, as the rectangle will also include points that are a bit more far away.
2. You want to filter OSM data:
You might use a reverse-geocoding service as Nominatim to get informations about points of interests that are within this distance: http://wiki.openstreetmap.org/wiki/Nominatim
Otherwise import the OSM data using osmosis to a PostGIS DB. AFAIK there is (currently) no DB tool for Oracle: http://wiki.openstreetmap.org/wiki/Oracle
I'm sorry if I missed your question, but then please add more details :)