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#1 2020-04-27 09:39:29

ToGoGo
Member
Registered: 2020-04-27
Posts: 3

Washington, DC Zip Codes and Neighborhoods are incorrect

Hi Everyone,
I just joined up on OpenStreetMap. I read through the beginner's guide and am trying to avoid posting questions that have already been answered, so forgive me if this is the case in this instance. I am using OpenSteetMap API for a new non-profit online ordering and delivery platform for DC restaurants (ala Grubhub or UberEats, except we don't steal restaurants' money). We can't afford using Google Maps API and we thought this would be a good solution but there are some problems with the zip codes and neighborhood functionality in DC.

For instance, if you try to pull up an address on the 700 block of Harvard St NW, it shows two different neighborhoods (Pleasant Plains is correct, Chinatown is very wrong) and the incorrect Zip code (20009 instead of 20001). The Zip code issue I think I fixed, but I don't know how to fix the neighborhood issues.

Can someone please steer me in the right direction?

Thanks for your help and everything you all are doing to improve this tool. We're going to try to put it to good use and help some businesses get back on their feet.

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#2 2020-04-27 10:53:53

user_5359
Member
From: Königswinter
Registered: 2008-12-25
Posts: 311
Website

Re: Washington, DC Zip Codes and Neighborhoods are incorrect

ToGoGo wrote:

For instance, if you try to pull up an address on the 700 block of Harvard St NW, it shows two different neighborhoods (Pleasant Plains is correct, Chinatown is very wrong) and the incorrect Zip code (20009 instead of 20001). The Zip code issue I think I fixed, but I don't know how to fix the neighborhood issues.

Did you mean this street https://www.openstreetmap.org/way/130764273 ? Please have a look on the value tiger:zip_left = 20001 and tiger:zip_right = 20009.
Only the houses on the right have a zip code (addr:postcode=20001). The street above (Hobart Place North, https://www.openstreetmap.org/way/6056624) and below (Gresham Place Northwest, https://www.openstreetmap.org/way/6054391) have booth the zip_%=20001. It seems better to me to correct the errors in the data than to correct the results of nominatim. But I myself do not have sufficient knowledge of the local conditions.

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#3 2020-04-27 11:11:17

SK53
Member
Registered: 2009-01-11
Posts: 649

Re: Washington, DC Zip Codes and Neighborhoods are incorrect

Basically you need to look at the detailed output from Nominatim (which is not the OSM API, its the API to a geocoder using OSM data).

This is 775 Harvard St NW: https://nominatim.openstreetmap.org/det … s=building,
and this is the underlying OSM object: https://www.openstreetmap.org/way/69204 … /-77.02531

So some things to note:

* The building does not have the zipcode on it, therefore the zip code has to be computed
* You can see the entry for computed zipcode in the breakdown of the Nominatim
* Pleasant Plains is mapped as an area with type place=neighbourhood
* Chinatown is mapped as a node with type:place=suburb

This means that not only are the bounds of the zipcode computed, but so are the bounds of Chinatown. As it is marked as a suburb it is assumed to consist of several neighbourhoods. It looks as though there are no other suburbs mapped in DC N of Chinatown so all of them will end up being allocated to Chinatown. You can view the neighbourhoods and streets which belong to Chinatown here https://nominatim.openstreetmap.org/det … lass=place.

There are a few more complex things going on: Columbia Heights is marked as both a neighbourhood with fixed boundaries and as a suburb but appears to be only treated as a neighbourhood.

Zipcodes are probably derived from Tiger data which is retained on street geometries (https://www.openstreetmap.org/way/13076 … /-77.02411) and you can see Harvard St NW has 20001 on one side and 20009 on the other which likely makes computation harder.

Summary:

* zip codes have not been added explicitly to buildings and are therefore computed. I'm not sure exactly how this works, but accurate postcodes on individual addresses is best practice
* neighbourhoods and suburbs have been added inconsistently around Washington DC. I'm not sure of the reasons for this, but suspect someone wanted certain places to show up on earlier than others on the map. (This is a bad thing, termed "Mapping for the Renderer": bad because it usually produces unwanted side effects for other applications such as geocoding).

What can you do about it?

It really depends on how you want to use this data. I'm assuming you expect users to search for a restaurant and you want to provide a street address, locality, zip format for viewing, so a reasonable first approximation would be to use only the first place element after the street address. I think the API provides JSON & GeoJSON outputs which make parsing a bit easier, i.e, only work with 775 Harvard St NW, Pleasant Plains, Washington DC, <zipcode>.

You can, of course, work to improve the data. However, please ensure that you do not use sources such as google etc. If the zipcodes encoded in the street geometries are accurate these can be directly added to the addresses along the street (e.g., using addr:postcode).

For more detailed help there is a nominatim mailing list, IRC channel, and OSM-US hosts a Slack channel.

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#4 2020-04-30 08:58:52

ToGoGo
Member
Registered: 2020-04-27
Posts: 3

Re: Washington, DC Zip Codes and Neighborhoods are incorrect

SK53 wrote:

Basically you need to look at the detailed output from Nominatim (which is not the OSM API, its the API to a geocoder using OSM data).

This is 775 Harvard St NW: https://nominatim.openstreetmap.org/det … s=building,
and this is the underlying OSM object: https://www.openstreetmap.org/way/69204 … /-77.02531

So some things to note:

* The building does not have the zipcode on it, therefore the zip code has to be computed
* You can see the entry for computed zipcode in the breakdown of the Nominatim
* Pleasant Plains is mapped as an area with type place=neighbourhood
* Chinatown is mapped as a node with type:place=suburb

This means that not only are the bounds of the zipcode computed, but so are the bounds of Chinatown. As it is marked as a suburb it is assumed to consist of several neighbourhoods. It looks as though there are no other suburbs mapped in DC N of Chinatown so all of them will end up being allocated to Chinatown. You can view the neighbourhoods and streets which belong to Chinatown here https://nominatim.openstreetmap.org/det … lass=place.

There are a few more complex things going on: Columbia Heights is marked as both a neighbourhood with fixed boundaries and as a suburb but appears to be only treated as a neighbourhood.

Zipcodes are probably derived from Tiger data which is retained on street geometries (https://www.openstreetmap.org/way/13076 … /-77.02411) and you can see Harvard St NW has 20001 on one side and 20009 on the other which likely makes computation harder.

Summary:

* zip codes have not been added explicitly to buildings and are therefore computed. I'm not sure exactly how this works, but accurate postcodes on individual addresses is best practice
* neighbourhoods and suburbs have been added inconsistently around Washington DC. I'm not sure of the reasons for this, but suspect someone wanted certain places to show up on earlier than others on the map. (This is a bad thing, termed "Mapping for the Renderer": bad because it usually produces unwanted side effects for other applications such as geocoding).

What can you do about it?

It really depends on how you want to use this data. I'm assuming you expect users to search for a restaurant and you want to provide a street address, locality, zip format for viewing, so a reasonable first approximation would be to use only the first place element after the street address. I think the API provides JSON & GeoJSON outputs which make parsing a bit easier, i.e, only work with 775 Harvard St NW, Pleasant Plains, Washington DC, <zipcode>.

You can, of course, work to improve the data. However, please ensure that you do not use sources such as google etc. If the zipcodes encoded in the street geometries are accurate these can be directly added to the addresses along the street (e.g., using addr:postcode).

For more detailed help there is a nominatim mailing list, IRC channel, and OSM-US hosts a Slack channel.

Thank you so much for your response. This makes much more sense now. I have no idea why Chinatown would be marked as a suburb considering it is a neighborhood like any other in DC. Can I delete it as a suburb? I don't want to jump on the platform and mess things up, but this is definitely incorrect. I really appreciate the help.

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#5 2020-04-30 09:05:20

ToGoGo
Member
Registered: 2020-04-27
Posts: 3

Re: Washington, DC Zip Codes and Neighborhoods are incorrect

Did you mean this street https://www.openstreetmap.org/way/130764273 ? Please have a look on the value tiger:zip_left = 20001 and tiger:zip_right = 20009.
Only the houses on the right have a zip code (addr:postcode=20001). The street above (Hobart Place North, https://www.openstreetmap.org/way/6056624) and below (Gresham Place Northwest, https://www.openstreetmap.org/way/6054391) have booth the zip_%=20001. It seems better to me to correct the errors in the data than to correct the results of nominatim. But I myself do not have sufficient knowledge of the local conditions.

Thank you for your help. It looks like the zip codes were set up incorrectly for the neighborhood. Does it work like geofencing? Sorry, my understanding of map building is extremely minimal, I'm just getting started.

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