I am trying to retrieve a set of static map tiles from Google Maps using the simple web services API.
For zoom level 0, where there is only one tile, I simply specify the center as (0,0), which is trivial. (And yes, I am aware, that the south-north range is [-85.05113°,85.05113°] only, i. e. I have read about Mercator projection (not only the Wikipedia article).
So for zoom level 1 with 2×2 tiles I know that it'd be naive to think the central coordinates of the tiles would be (±90,±45). I applied one of the functions listed in above article's "Derivation of the Mercator projection" section. In PHP this looks like
function mercor($aLat) {
return rad2deg(asinh(tan(deg2rad($aLat))));
}
which always returns 50.498987 when passing in 45. I tried several of the alternative expressions :-) all with the same result, indeed, and even checked that value with a dedicated calculator.
Anyway, the value is obviously not correct, as you can see here (which was supposed to show the north west quadrant of the Earth's map, but here the equator is shifted to the north) (image source):
By trial and error I found the correct value to be close to 66.65, but I have no idea how to calculate that. I tried several manipulations of the R factor shown in the expressions in that mentioned "Derivation of the Mercator projection" section, and also fiddled with the "Scale factor" described in the same document, but whatever I do, I do not come close to 66.65.
Here you can see that 66.65 is the (close to) correct value, the equator at the bottom edge (image source):
And taking an illustration like, for example (image source):
and a tool like kruler, it also becomes obvious that it should be 66.65.
So, any idea would be greatly appreciated. Thanks in advance...
(Currently I'm trying it from withing HTML documents, because it's relatively simple, but later I'd like to do it from an environment in which I have no way to call JavaScript. so the JavaScript API is not an option.)
I have prepared a website describing the popular tiling system used behind Google Maps, OSM and similar slippy maps. Have a look at:
http://www.maptiler.org/google-maps-coordinates-tile-bounds-projection/
With a click on the interactive map you receive bounds calculated in both the Mercator coordinates (in meters) as well as WGS84 lat/lon (in geodetic degrees).
Down on the page you find an open-source python code (which has been in the meanwhile ported to other programming languages by other people). This contains documented description of all the math you may need.
In fact, I have created this resource after working for Google SoC on the GDAL2Tiles/MapTiler project - which has been reimplemented into C/C++ and significantly improved and is now available via http://www.maptiler.com/.
UPDATE:
If you want to use the Google Maps Static API requests you may find useful another online tool which I prepared: http://www.maptiler.org/photoshop-google-maps-overlay-tiles/.
Feel free to review the JavaScript source code.
Related
Background
We are supplied with some AIXM data (an XML based superset of GML) which describes polygon areas on a map as a mix of GeodesicStrings (a list of coordinates) and ArcByCentrePoints (a centre point coordinate with a radius, start bearing and end bearing). We are taking this data and converting it into a simple list of coordinates that we then display using a Google maps polyline.
Problem
When we plot a shape with an arc, the start and end points of the arc usually don't match up with the end point of the preceding line and the start point of the subsequent line. It looks as if the radial distance is out by an amount which doesn't appear to be proportional to the radius. See screenshot: interestingly the smaller arc at the top seems fine but the larger arc is inset.
We're pretty sure the data is correct because it looks fine when we use a third party tool to visualise it, so we're doing something wrong.
Implementation
We are using the turf.js library to convert the arc description into a set of points using their lineArc function. Internally this utilises their destination function which "uses the Haversine formula to account for global curvature". We combine these generated points, in the correct sequence, with the points taken directly from the preceding and subsequent GeodesicString elements to give us our final polygon.
Data
Input: Fragment of AIXM (GML) describing polygon
Output: Resulting list of points
Help!
I'm aware this question is light on code but I hope I've described the problem adequately and that some kind person with more GIS knowledge than me (>0) might be able to point me in the right direction. Thanks :)
I've given a couple of presentations on debugging and one of the things I say is that you should keep an open mind and shouldn't get too fixated on a possible cause of a bug because you can waste a lot of time tracking down a false lead.
Sadly in this case I didn't take my own advice. I was so obsessed with the idea that the problem arose from a complex cause, such as issues with the implementation of the Haversine formula, that I overlooked the far simpler answer. My code was taking a string representation of the radius, including the units (e.g. nautical miles or meters) and converting it into kilometres. Sadly I was using parseInt rather than parseFloat as part of this and so instantly losing precision. It was a simple as that - a schoolboy error.
Big thanks to Stefano Borghi, a maintainer of Turf JS, for all his help with this and for helping me see the wood for the trees.
I want to create an online map for a hiking trail, and I have been using my smartphone to collect coordinates from it.
I have many questions regarding what's good practice when it comes to making such tracks, but for starters: it would look much neater if I could edit the readings so that they are right on top any roads in Google Maps. How can I achieve this?
EDIT: I want to find the coordinates that will make a track along a road look exactly as if I'm using snapping/directions even when I'm not.
I've tried tools such as QLandkarte GT and Viking, and with the latter I was able to manually remove excess coordinates and move the remaining ones so that they're exactly on top of roads (inside Viking, using OpenStreetMap). However when I load the edited .kml-file in Google Maps, the roads seem to be in slightly different places and the result is hardly better than before editing.
I tried using the hack that exports GMaps directions as .gpx, thinking I could insert Google's own coordinates along the roads. But the exported file only had coordinates at the turning points with straight lines between these.
QLandkarte GT supposedly has a snap-to-road feature (see answer in link), but I haven't found it and I also don't know how to obtain a vector map of the area.
Then there's Google Earth which people seem to use for this, but again I can't find any track editing features (in the free version).
In short:
How do I edit my existing tracks so that they match roads in GMaps...
OR
How can I obtain new tracks representing roads in GMaps...
...so that the resulting track is as smooth as Google's own directions or other professional GPS-data, when displayed in GMaps?
It seems like it's possible to get these coordinates from Google Maps after all, exporting the directions polyline. I'm not sure how, it seems to involve adding the parameter output=dragdir to the link.
Anyway there's a tool called GPS Visualizer that can create a .kml-file with the wanted coordinates.
Edit: Will accept my own answer as long as it's the only one, but I would still like to know more.
I’m wondering if one of you can point me to a Google Maps API V3 code example which solves the following problem:
I have a data set with two-dimensional point data which is NOT georeferenced; i.e., the data do NOT refer to locations on the surface of the earth. In my case the data are for documents located in an artificial/synthetic, two-dimensional ‘information’ space.
For the moment I am successful in displaying and interacting with these points in the Google Maps API, mapping all points as long/lat coordinates centered on the long/lat coordinate 0:0; i.e., somewhere in the Atlantic Ocean off of the West coast of Africa :-)
Although I can get away with this for experimental purposes (the ocean around 0:0 is pretty much empty, so it does not interfere with my visualization), I’d like to adopt a new, abstract coordinate space; essentially replacing Google’s ROAD/TERRAIN/etc. maptype with my own (empty) one.
From what I have googled, this can indeed be done and I have seen some impressive DNA/genomics work in which this has in fact been done. But rather than hacking those complex apps, I’d like to see a very simple, base code example where someone plots some arbitrary points in an arbitrary space using the Google API.
Can one of you point me to such an example?
All,
I appears that the answer was in front of me all along (it usually is... sigh).
It seems that developers.google.com gives a nice example at
https://developers.google.com/maps/documentation/javascript/maptypes#CustomMapTypes.
The code for the example itself is at https://developers.google.com/maps/documentation/javascript/examples/maptype-base
Seems like this is basic enough for me to start working from.
It's any public maps API (google, microsoft, openstreet...) that returns the height of the roof for certain points?
Imagine: you request some address and the API returns the coordinates, some information and the height of the building, if available.
OpenStreetMap has a height tag and various other building related tags like building:levels from which you could estimate the height. However note that currently most buildings lack those tags as they aren't widely in use so far. Still you could give it a try and request all buildings around a specific point or area using Overpass API / overpass turbo and then look for those tags.
I'm almost certain the answer is "no". To get this information you'd need LIDAR data or something similar, and there is no global dataset of LIDAR data at anything like the resolution necessary to accurately determine the height of an individual building roof.
Many of the web-mapping services use terrain and building elevation models internally to calculate their "3d" view modes, but I don't think this data is ever exposed, and even if it were I think it's only suitable for display purposes rather than for performing any calculations.
I need to find the width of roads in Google Maps.
Google Earth can't be used as it doesn't support Linux.
Streetviews cannot be used, since it is not available in the area I live.
This link: http://osdir.com/ml/google-maps-js-api-v3/2011-05/msg00666.html suggests the method of finding the distance between the two ends of the road by knowing the coordinates of both sides.
Therefore the first question is as in title.
BTW, I have managed to display the "route" between the two coordinates already on the maps.
The type of the map can be set to "satellite" view through the API.
Does that affect the distance and coordinates that'll be fetched?
All this stuff about getting points across the street from each other is further complicated by the fact that sometimes the geocode returned will be a ROOFTOP geocode--likely meaning the centroid of a building--and other times, it is RANGE_INTERPOLATED which suggests that it will be not very precise. See http://code.google.com/apis/maps/documentation/javascript/reference.html#GeocoderLocationType. In either case, it won't be the edge of the road.
It may be sobering to look just how far from the road Google Maps may put the marker for an address: http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=4+Clinton+Drive,+Englishtown,+NJ&aq=&sll=40.299985,-74.290066&sspn=0.009426,0.015213&gl=us&ie=UTF8&hq=&hnear=4+Clinton+Dr,+Englishtown,+New+Jersey+07726&t=h&z=16
In short, unless your use case can tolerate a pretty wide error bar, I think it's safe to say that this just isn't going to work, at least not without considerable resources at your disposal.
Google Earth web plug-in is not available for Linux, but if you don't need it to be a web page, then you can try working with Google Earth app, perhaps. It works on Linux.
If your use case is for a reasonably narrow geographical area, there may be data available elsewhere. But getting this from Google Maps API v3 is unlikely to be a very good option.
(As always when I give a "I don't think this is feasible" answer, I'd be happy for someone to come along and prove me wrong.)
I'd look at the coordinates of individual houses. If house numbers are arranged such that n+1 is on another side than n, the distance between the two might tell you something about the width. But I don't think there's an API for that...
Here's the NON API solution i thought of,
In my country , roads named with different code names like "A3" , "B354", "C6".
"A" is for main roads. Most of the time width of the road is consistent as per road code. Therefore we can use road code to get the width of the road. I don't know if this works for your country.