<div dir="ltr"><span style="font-size:12.8000001907349px">Hey Stefan!</span><div style="font-size:12.8000001907349px"><br></div><div style="font-size:12.8000001907349px">First of all thanks for writing, I'm not involved with the nominatum project but I may be able to help out with some geographic concepts.<br><br>To answer your first point, yes, "reverse geocoding" is generally considered to be a process of asking the database for a list of the geographically nearest database entries to the co-ordinates you entered, it could return one or more result and optionally sort those results by distance from the input co-ordinates.<br><br>This process works really well for areas of the world which have data points (like venues, places, buildings, roads etc.) but will not work so well for places which don't have many "points", such as in the middle of a desert or ocean.<br><br>A good example of this are the co-ordinates 0,0 which land in the middle of the Altantic Ocean south of Ghana and east of Gabon, the nearest land mass is hundreds of kilometres away (despite a running joke that there is an island there called null island).<br><br>So when you ask nominatum to reverse geocode the 0,0 co-ordinates you get back an error "unable to geocode" which simply means "I couldn't find a real world 'thing' near here, so instead of giving you something very very far away, I returned nothing"<br><br>Regarding your question about Italy and the co-ordinates 40,18. I checked and 40,18 actually lies in shallow water very close to land just south of Gallipoli on the very Eastern part of Italy and it indeed due South of your Slovakian co-ordinates of 48,18.<br><br>One of the problems with reverse geocoding countries, cities, states etc. is that they don't really have a "true" center point, in the database they are represented by a set of points (vertices on it's border) and are grouped together as a polygon. Due to border disputes and historical politics those geometries can be very unusual shapes and I'm not aware of any place in the world which fits a nice simple shape profile like a rectangle or triangle.<br><br>To give you a bit more of an idea what I'm talking about I did a super quick trace of your country by clicking points along the border line I can see on the visual basemap which resulted in this very simplified outline:<br>[link to border data for slovakia] <a href="https://gist.githubusercontent.com/anonymous/5e29ce69e99981828c03/raw/f9b1174cb40f50a99e5c608dfeda89a6c2e8f6f3/map.geojson" target="_blank">https://gist.githubusercontent.com/anonymous/5e29ce69e99981828c03/raw/f9b1174cb40f50a99e5c608dfeda89a6c2e8f6f3/map.geojson</a><br><br>That data format is called geojson (you can find more about it on the net) and it's reasonably safe to say that any point that lies inside that shape is somewhere inside <span style="font-family:Arial">Slovakian national borders.<br><br></span>These geometries can be found online and I can link you if you are interested, however they can be very detailed and very complex, meaning that they are very difficult to deal with unless you are an experienced software engineer. Simplified versions of these geometries exist (as per lossy audio codecs), which obviously lose fidelity but are useful for things like rendering visual map layers.<br><br>Sorry for the long wordy reply, I just wanted to explain this concept so that you understand that there also exists the idea of "point inside polygon intersection" which means that you can ask the database to tell you which polygons envelope the co-ordinates you specify, this is a bit different as it does not require a place or 'thing' to exist in the database in order to return a result. I'm not sure if nominatum offers this functionality.<br><br>In regards to your second question, you are correct about your co-ordinates being in the <span style="font-family:Arial">Adriatic sea at </span><font face="Arial">41,18 and also probably why the results you saw differed from those you got when your co-ordinates were over land. I am assuming that this is because the Italy polygon data includes the </font><span style="font-family:Arial">Adriatic sea area.</span><br><br>In short answer to your question, reverse geocoding and nominatum will be very useful for you when working on land, however when working with oceans you might be better off finding something else like the overpass API.<br><br>I'll leave it for other people more experienced in nominatum and openstreetmap to suggest more tools to look at.<br><br>All the best, I've been to your country before for the Pohoda Festival and had a great time.</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Jul 7, 2015 at 11:48 PM, Štefan Kiss <span dir="ltr"><<a href="mailto:stefan.kiss@nextra.sk" target="_blank">stefan.kiss@nextra.sk</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><u></u>
<div bgcolor="#ffffff">
<div><font size="2" face="Arial"> Hello everybody!</font></div>
<div> </div>
<div><font size="2" face="Arial"> My name is Stefan and I am from Slovakia. I
am a blind user of computer and for the work with PC I am using a screenreader
which cann me to read a text from screen. I cann work with text, e-mail,
webpages, sound but not with pictures and maps. But I am interested in geography
and I would like to find some possibility for reading maps.<br> I found a
Nominatim service and I thought that it is a chance for me. I am living in
Slovakia (48 lat and 18 lon) and I sent to Nominatim a sequence of queries where
lat was 48 and lon was step by step decremented from 18 to 0. It was great for
me to know what about countryes are under my Slovakia. I think - this is a way
for blinds - how they cann read a maps and to create some image about the world.
<br> But now I hawe 2 questions to you. </font></div>
<div> </div>
<div><font size="2" face="Arial">1. On the webpage of Nominatim service is
written:</font></div>
<div> </div>
<div><font size="2" face="Arial">"...The following examples use Nominatim to reverse
geocode. This is the process where you begin with a geographic coordinate and
the nearest known address is returned."</font></div>
<div> </div>
<div><font size="2" face="Arial"> Please, what it means? When I send to
Nominatim a coordinates 40 18 <br>(<a href="http://nominatim.openstreetmap.org/reverse?format=xml&lat=40&lon=18&zoom=18" target="_blank">http://nominatim.openstreetmap.org/reverse?format=xml&lat=40&lon=18&zoom=18</a>)<br>and
I get result Apulia, Italia, means it that I am really in Italia or the result
is only the nearest known address and my location 40 18 dont must be in
Italia?</font></div>
<div> </div>
<div><font size="2" face="Arial">2. When I sent to Nominatim a sequence of queries
and I went from 48 18 down to 10 18, I saw, that the seas are missed. I know
that under Europe is Adriatic sea but in Nominatim results I go from Italia to
Admas Tisal and Libia. (Admas and Tisal are tagget with tag address29 without
other tags about Country or city).</font></div>
<div> </div>
<div><font size="2" face="Arial"> Please, cann you recommend me how specify
the query when I would like to have a really information about place which is
appointed with lat and lon coordinates? Or if Nominatim is not the best
way for me, do you know some service which cann I use for reverse geolocation? I
dont need roads, houses, buildings or points. I would like browse the maps
through gps coordinates and to know in which country or city or on which sea I
am. Is it so unrealistically?</font></div>
<div> </div>
<div><font size="2" face="Arial"> Thanks</font></div>
<div> </div>
<div><font size="2" face="Arial"> Stefan from
Slovakia<br></font></div></div>
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<br></blockquote></div><br><br clear="all"><div><br></div>-- <br><div class="gmail_signature"><div dir="ltr"><div style="color:rgb(136,136,136)">Peter Johnson</div><div style="color:rgb(136,136,136)"><font face="arial, sans-serif">Senior Geo-Search Engineer</font><br></div><div style="color:rgb(136,136,136)"><a href="mailto:peter.johnson@mapzen.com" style="color:rgb(17,85,204)" target="_blank">peter.johnson@mapzen.com</a></div><div style="color:rgb(136,136,136)"><a href="https://github.com/missinglink" target="_blank">https://github.com/missinglink</a></div></div></div>
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