I'm working with a maps application, One of the functionality of this app is to allow the user to draw a polygon on the map for specific task.
This Polygon will be save on our servers for later use.
I managed to encode the points and produce an encoded string path to reduce the size of storing and transferring the polygon from and to the server.
String path = PolyUtil.encode(latLngs); // Android
But I read on Google developer webiste that:
Polyline encoding is a lossy compression algorithm that allows you to store a series of coordinates as a single string. Point coordinates are encoded using signed values. If you only have a few static points, you may also wish to use the interactive polyline encoding utility.
I get a shock when read this :( because I use encodedPath in more than one App.
What does they mean by lossy compression ?
So is it safe to use this type for storing users' polygons ?
I mean: is it true that every decode of an encoded string will produce the same points ?
Or it is better to store the polygon on the server as multi points ?
Thanks for any help.
If you take a look at the encode method that is used to encode a sequence of latitude/longitudes on the Google Maps Android API Utility Library you will see that it rounds the fifth decimal place:
long lat = Math.round(point.latitude * 1e5);
long lng = Math.round(point.longitude * 1e5);
From the Wikipedia, 0.00001 decimal degrees is equivalent to 1.1132 meters at equator.
Anyway, take into account that this is the behaviour now, and it's transparent to you, so you may not notice if the implementation changes to allow more precision or if the precision is more truncated. Thus, if the precision of the coordinates is important to you, you may want to store the coordinates as a list of points on your system.
Related
I've been checking out the documentation for Google's "My Maps" service, and I can't figure out if it will let me do what I need to do.
Our client has a map, created using My Maps, that defines several delivery areas using colored polygons. I understand that it's possible to export a KML file containing the coordinates for these polygons, and that it's also possible to create a network link so said KML file will be updated whenever the map is.
My problem: I don't know what to actually do with the KML file. I need some sort of script that can take the network-linked KML, and return a human-readable set of coordinates that I can use to define delivery areas on the client's site. In other words, they need to be able to edit or create delivery zones on this map, then on their site, bind each polygon to a particular store to determine which store must deliver which orders. There's a lot of complicated stuff going on here, but for this first step I just need to know how to get the coordinates for the separate shapes in a reliable way.
I wish I had some code to post to make this a more useful post, but it's more a question of whether or not this type of functionality is actually possible. Please help me figure this out!
Thanks for any and all help.
The KML file is a XML file then you can inspect the file both server side with a XML parser or client side via JQuery.
this is an essential javascript sample for a client side approch:
var request = new XMLHttpRequest();
request.open("GET", filename, false); // filename = kml_filename
xmlDocObj = $($.parseXML(request.responseText));
var placemarks = xmlDocObj.find("Placemark");
placemarks.each(function (index) {
... your parsing for the geometry object and coordinates content you need
}
in graphhopper the points are encoded then decoded in javascript
but i'd like to have the points not encoded, how is it possible ?
is there a php alternative of the decodePath(encoded, is3D) function ?
thanks
There is only a Javascript and Java version at the moment in GraphHopper but it shouldn't be hard to write one in PHP. Or see the docs on how to disable encoded polylines.
I developed an app that sends GPS points to a server and the server convert those GPS points into a path, which is shown in a map.
Now the server makes a call to Google Maps Directions service and gets a response. From that response I use the value overview_polyline, which based on the documentation:
Contains an object holding an array of encoded points that represent
an approximate path of the resulting directions.
The problem is that sometimes the line I get is really weird. Sometimes it gives me a path where there's no street at all.
Here are some screenshots to describe my problem. The yellow marker is the start point (A), and the blue marker is the end point (B).
So I asked Google to give a route from A to B, with driver mode, so it shows the actually path.
And this is the result:
This is the API call I use. My question is: Any idea why would it return that path?
http://www.geocodezip.com/v3_example_geo2.asp?addr1=53%20Imperial%20Avenue,%20Westport,%20CT%2006880,%20USA&addr2=2%20Harborview%20Road,%20Westport,%20CT%2006880,%20USA&geocode=1&geocode=2&type=m
Looks to me like the encoded polyline is not being rendered correctly. How is your code handling that?
example using the geometry library decodePath function
Looks to me like Google broke their polyline encoding.
If I paste it into here (which admittedly doesn't expect polylines, it expects polyons)
I need to escape the "`", change:
{pazFraw~L|#|#f#^`#N\\?XIx#a#n#[
to:
{pazFraw~L|#|#f#^\`#N\\?XIx#a#n#[
to make it have a more reasonable shape.
I want Google Maps markers that will:
Change on a daily basis
Be loaded depending on (if there is an event at particular venue tonight)
Loaded Categorically (can be turned on/off)
loaded by date
I expect to have an average of 40 markers on screen at a time, and max no more than 300.
My markers are sorted in an array. What should I go with?
I'm not entirely sure I understand your question but hopefully I can help.
I am presuming this is part of a larger website with an existing db behind it so it sounds to me that you should store your markers in the db and read the required markers out when the page is requested. Your business logic will make sure you get the correct markers at any given time. Your view can then take an IEnumerable which you can use to populate your map.
Your Marker class can look something like this
public class Marker
{
public decimal Longitude { get; set;}
public decimal Latitude { get; set;}
//...plus any additional info you want to add (maybe text for each marker)
}
You've probably already come across this but, if not, here is a good place to get info on using the google map api:
http://code.google.com/apis/ajax/playground/#markers_v3
Apologies if this is not helpful as I have had to make a few assumptions on what you are working with.
My understanding of XML is more of a data transport mechanism rather than storage. Typically I'd store everything in the database and use XML or JSON to move data around.
My approach would be to store all the relevant information in a database and have a script that receives a request (it can include parameters such as date, category, etc) and returns the relevant rows as JSON (thought this can be XML or any other format).
The JavaScript code would be composed of a function that generates the request based on user input (or whatever data you need to get) and once it receives passes the data to a second function that creates the markers and the infowindows if neede
jQuery becomes very handy. You can use %.ajax()or $.getJSON() to make the requests and work with the data you get from the database.
This might help: http://code.google.com/apis/maps/articles/phpsqlsearch.html
I use such urls like:
http://maps.google.com/?saddr=546%206th%20Avenue,%20New%20York,%20NY%2010011%20(Sixth%20Avenue,%20New%20York)&daddr=W%20103rd%20St,%20New%20York,%20NY%20&dirflg=r
But it returns full html page. Does anybody know how to get such info in json or xml or any parsable format?
I need public transit.
You just add &output=json to the end of the url.
I'd like to extend Mathias's answer a little bit.
There's no official Google Transit API at the momemnt. Transits are provided by agencies, and most of Transits are not public. So, Google is not allowed to open them as API.
You may try to consume the "unofficial" data using your link + "&output=json".
However, the result won't be a valid JSON. Instead, that's something, that can be easily converted to a JavaScript object. (The differences are: there is no quotes around property names,
the strings are not properly encoded etc.)
Imagine you got this JavaScript object. However, it won't allow you to easily get the structured route details. Object's properties contain the route points coordinates, but no descriptions. The only place where the descriptions may be found is 'panel' property, which contains a chunk of HTML text (you may find a link to the sample of HTML in my blog post)
So, you'll have to convert this HTML into XML (X-HTML) and then build the parser of this XML to get the essence data of a trip.
Seems like a bit of overkill to me. Having in mind, that "unofficial" API may change in the future, including slight changes in 'panel' HTML structure that will kill your parser.
#MathiasLin, how did you overcome this?
Google Maps Javascript v3 and the Directions Web Service now has this capability as per :
https://googlegeodevelopers.blogspot.in/2012/06/public-transit-routing-and-layer-now.html
Update:
Latest server side API docs:
https://developers.google.com/maps/documentation/directions/get-directions
Re choosing public transport - it seems that it defaults to public transport now (at least when I use it).
Changing the last URL parameter, dirflg=r, into dirflg=w switches the directions to walking mode.
Plus see:
Walking, bicycle and public transport directions with time required in each mode , iPhone
for more detail about the parameters.
Google direction API response is in HTML, JSON format please check https://developers.google.com/maps/documentation/javascript/directions?hl=lv
& section 'The DirectionsResult Object'