Unexpected token < in JSON at position 0 in overpass api request

Question

I'm using overpass api to fetch informations about hospitals in a certain area. This is the url I use to get the data:

https://overpass-api.de/api/interpreter?data=[out:json][timeout:25];(node[amenity=hospital](41.862018386633096,12.380561828613283,41.9435317956981,12.612133026123049);way[amenity=hospital](41.862018386633096,12.380561828613283,41.9435317956981,12.612133026123049););out;

Since the hospitals can be also way other than simple nodes, I have to treat them differently so I do this:

fetch(url).then(response => {
return response.json();
}).then(results => {
 results.elements.forEach(e => {
  console.log('Name '+e.tags["name"])

  if(e.type == "way"){
      console.log('ID: ' + e.id)
      let urlWay = `https://overpass-api.de/api/interpreter?data=[out:json][timeout:25];(way[amenity=hospital](${e.id}););out%20center%20meta;`;
      fetch(urlWay).then(response=> {
          return response.json()
      }).then(result => {  
        lat = result.elements[0].center["lat"]
        lon = result.elements[0].center["lon"]
      })
    } else {
      //get lat lon info normally 
     }

Then I use lat and lon to put a marker on the map.
The explanation for this is that a "way" does not have a single lat-lon information, but it is formed by several nodes. This is an example:

{
"type": "way",
"id": 107248762,
"nodes": [
1233017145,
1233017732,
1233017520,
1233017183,
1233018290,
2144891055,
1233017895,
1233017145
],
"tags": {
"amenity": "hospital",
"wheelchair": "yes"
 }
},

So, to get the lat lon informations, I use the "id" property to get the center of the building, by using the urlWay in the code above, specifying out center meta. I use elements[0] because the array has only one element. You can see an example of output by clicking here.
The problem is that, at some point while fetching the informations about the ways and putting markers on the map, the program stops and gives me the error:

SyntaxError: Unexpected token < in JSON at position 0 in overpass api request

I'm assuming (don't know if I'm right) that there is one of these JSON files I get that is not correctly written, so I'm wondering how can I prevent this exception, maybe by just ignoring a bad formatted file.

EDIT: I've modified the code as proposed by user mmd in the answers and worked correctly, so it probably was an error linked to rate limiting.

if you go to the network tab of your browser's debug tools, you can see the actual response of the request to help you see what's going on. There are strong chances that the script you are calling is responding an html error or warning page, so that first char is a `<`. Happens to me a lot — Kaddath, Aug 23 '17 at 10:44
Can you provide a real [mcve]? You've [got syntax errors](http://jsbin.com/gifuwopewi/1/edit?js,console) in the code you shared and expecting everybody who wants to try out your test to case fix them themselves is unreasonable. — Quentin, Aug 23 '17 at 10:49
The problem is that I don't know which of the calls for the different IDs is giving the error, so I'm looking for a way to avoid this error, by maybe ignoring those file a priori. @Quentin I'll modify the question. — Usr, Aug 23 '17 at 10:51
tip: you could add a `try ... catch` around the whole `fetch` or around the `response.json()` to console.log the url causing the error — Kaddath, Aug 23 '17 at 11:58

mmd · Accepted Answer · 2017-08-23T19:54:53.963

The issue is most likely related to rate limiting or other error conditions, in which case you receive an XML formatted error message, albeit the HTTP status code might still be HTTP 200! This message cannot be parsed as JSON string, hence the error message.

In general, your approach to query Overpass API one-by-one is quite inefficient. Instead you can also provide several Ids in one go. It is highly recommended to bundle requests this way, rather than sending one request for each single ID, which severely increases chances to hit rate limiting.

way(id:1000,1002,1003,1004)[amenity=hospital]

In wonder, why you don't use out:center in your original query get get all details on one go.

[out:json][timeout:25];
(node[amenity=hospital](41.862018386633096,12.380561828613283,41.9435317956981,12.612133026123049);
way[amenity=hospital](41.862018386633096,12.380561828613283,41.9435317956981,12.612133026123049););
out center;

score 0 · Answer 2 · answered Aug 23 '17 at 10:59

0

After some time the api throws 429 Too Many Requests Error. Try addind some timeout to your api calls.

answered Aug 23 '17 at 10:59

Lisa Schuh

69
3

Unexpected token < in JSON at position 0 in overpass api request

2 Answers2