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County borders were originally supplied by the USGS. Those boundaries are not of great precision. Also, that import process created a lot of overlapping ways, and didn't use the admin_level=* key.

Much work has been done on the county boundaries. There is now a boundary relation for each county, and all county boundary ways have been split and merged as necessary so that only one chain of non-overlapping ways defines the boundary between any two adjacent counties. (Along Ohio's border, Ohio's county boundaries were merged with the TIGER-imported state boundary. Generally, the boundaries of counties in adjacent states have not been modified, and remain sourced from imprecise USGS data.)

There are still tasks to be done, however. Where county and city boundaries overlap, they should be merged; the merged boundaries should have admin_level=6, unless of course they're also part of the state boundary, which would be 4. The accuracy of the USGS-sourced boundaries can be improved, by following the endpoints of TIGER data or clues visible in the aerial imagery; these clues might include visible property lines, and especially natural waterways. Check USGS topo maps when in doubt.

Because the TIGER import was conducted county by county, roads ended up disconnected wherever they crossed county lines. Worse, where roads followed the county line (especially in northwestern Ohio), two duplicate roads were imported, each with names and route numbers specific to one side of the road. We've mostly finished fixing the road network along county lines, though some issues are still waiting to be found.

One of OSM's strengths is that all types of data coexist in the same database, allowing you to (for instance) find roads that run along county lines without having to conflate two separate databases. Ideally, wherever a county or municipal limit clearly follows a road, river, or edge of a landuse area, we should "weld" the boundary relation to the other feature. For example, where the Butler–Hamilton County line follows Fields-Ertel Road, there is a way way that belongs to the relation Butler County and relation Hamilton County boundary relations. It is joined to way Fields-Ertel Road at every node along the road. (In Potlatch, press F repeatedly to "follow" the road you're welding the boundary to.) Please do not make the road itself a member of the boundary relations.

Tasks and Progress
Code Name Relation Boundary Source* City Boundary Merging TIGER welding
ADA Adams relation 911255 KY = T; Bro, Hig, Pik, Sci = U
ALL Allen relation 911260 U
ASD Ashland relation 911247 U
ATB Ashtabula relation 350382 maritime = M; land PA = T; Tru, Gea, land Lak = U
ATH Athens relation 911269 U
AUG Auglaize relation 911251 U
BEL Belmont relation 911264 WV = T; Moe, Nob, Gue, Has, Jef = U
BRO Brown relation 163283 KY = T; Cle = Yt; Cli, Hig, Ada = U Cle
BUT Butler relation 104783 IN = T; Pre, Mot, partial War = U; partial War, Ham = Yt partial War, Ham?
CAR Carroll relation 911249 U
CHP Champaign relation 180915 Log, Uni, Mad, Cla = Yt; Mia, She = U Log, Uni, Mad, Cla
CLA Clark relation 180912 Chp, Mad, Gre = Yt; Mot, Mia = U Chp, Mad, Gre
CLE Clermont relation 104779 KY = T; Bro, Ham = Yt; War = U Complete Ham, Bro, KY
CLI Clinton relation 104780 Gre, Fay = Ty; Hig, Bro, War = U Gre, Fay
COL Columbiana relation 911253 PA, WV = T; Jef, Car, Sta, Mah = U
COS Coshocton relation 241687 Mus, Lic, Kno = Yt; Hol, Tus, Gue = U Mus, Lic, Kno
CRA Crawford relation 911262 U
CUY Cuyahoga relation 350381 maritime = M; land Lak, Gea, Sum, Med, land Lor = U
DAR Darke relation 911258 IN = T; Mer, She, Mia, Mot, Pre = U
DEF Defiance relation 911265 IN = T; Wil, Hen, Put, Pau = U
DEL Delaware relation 178339 YTt Fra Complete
ERI Erie relation 350378 maritime = MT; land Lor, Hur, land San = U Ott Ott?
FAI Fairfield relation 178391 Yt Complete
FAY Fayette relation 270616 Yt Complete
FRA Franklin relation 177915 YTt Mad, Del Complete
FUL Fulton relation 911263 MI = T; Luc = Yt; Hen, Wil = U Luc Luc
GAL Gallia relation 911268 WV = T; Law, Jac, Vin, Meg = U
GEA Geauga relation 911256 U
GRE Greene relation 104781 Yt Complete
GUE Guernsey relation 911270 U
HAM Hamilton relation 104784 KY = T; But, War, Cle, IN = Yt Complete Complete
HAN Hancock relation 911252 U
HAR Hardin relation 911267 Uni = Yt; Log, Aug, All, Han, Wya, Mar = U Uni
HAS Harrison relation 911248 U
HEN Henry relation 911257 Luc = Yt; Woo, Put, Def, Wil, Ful = U Luc Luc
HIG Highland relation 271300 Fay, Ros = Yt; Pik, Ada, Bro, Cli = U Fay, Ros
HOC Hocking relation 270614 Ros, Pic, Fai, Per = Yt; Ath, Vin = U Ros, Pic, Fai, Per
HOL Holmes relation 911250 U
HUR Huron relation 911254 U
JAC Jackson relation 911266 U
JEF Jefferson relation 911261 WV = T; Bel, Has, Car, Col = U
KNO Knox relation 185763 Cos, Lic, Del, Mrw = Yt; Ric, Asd, Hol = U Cos, Lic, Del, Mwr
LAK Lake relation 350372 maritime = M; land Atb, Gea, land Cuy = U
LAW Lawrence relation 911271 WV, KY = T; Sci, Jac, Gal = U
LIC Licking relation 178389 Yt Complete
LOG Logan relation 184059 Uni, Chp = Yt; She, Aug, Har = U Uni, Chp
LOR Lorain relation 350377 maritime = M; land Cuy, Med, Asd, Hur, land Eri = U
LUC Lucas relation 350384 land Ott, Woo, Hen, Ful, land MI = Yt; maritime = M Complete Complete
MAD Madison relation 177916 Yt Fra Complete
MAH Mahoning relation 911312 PA = T; Col, Sta, Por, Tru = U
MAR Marion relation 185761 Mrw, Del, Uni = Yt; Har, Wya, Cra = U Mrw, Del, Uni
MED Medina relation 911294 U
MEG Meigs relation 911311 WV = T; Gal, Vin, Ath = U
MER Mercer relation 911290 IN = T; Van, Aug, Dar = U
MIA Miami relation 911306 U
MOE Monroe relation 911293 WV = T; Was, Nob, Bel = U
MOT Montgomery relation 104782 Gre = Yt; War, But, Pre, Dar, Mia, Cla = U Gre
MRG Morgan relation 911302 U
MRW Morrow relation 185762 Kno, Del, Mar = Yt; Cra, Ric = U Kno, Del, Mar
MUS Muskingum relation 241688 Per, Lic, Cos = Yt; Gue, Nob, Mrg = U Per, Lic, Cos
NOB Noble relation 911300 U
OTT Ottawa relation 350380 land Luc = Yt; maritime = MT; land San, Woo = U Luc, Eri Luc, Eri?
PAU Paulding relation 911314 IN = T; Def, Put, Van = U
PER Perry relation 241689 Hoc, Fai, Lic, Mus = Yt; Mrg, Ath = U Hoc, Fai, Lic, Mus
PIC Pickaway relation 178392 Yt Complete
PIK Pike relation 911307 U
POR Portage relation 911288 U
PRE Preble relation 911259 IN = T; Dar, Mot, But = U
PUT Putnam relation 911301 U
RIC Richland relation 911309 U
ROS Ross relation 270615 Hig, Fay, Pic, Hoc = Yt; Vin, Jac, Pik = U Hig, Fay, Pic, Hoc
SAN Sandusky relation 911303 maritime = M; land Eri, Hur, Sen, Woo, land Ott = U
SEN Seneca relation 911297 U
SCI Scioto relation 911291 KY = T; Ada, Pik, Jac, Law = U
SHE Shelby relation 911292 U
STA Stark relation 911298 U
SUM Summit relation 911310 U
TRU Trumbull relation 911299 PA = T; Mah, Por, Gea, Atb = U
TUS Tuscarawas relation 911305 U
UNI Union relation 178021 Yt Complete
VAN Van Wert relation 911295 IN = T; Pau, Put, All, Aug, Mer = U
VIN Vinton relation 911308 U
WAR Warren relation 104778 Ham, partial But, Gre = Yt; partial But, Mot, Cli, Cle = U Ham, partial But
WAS Washington relation 911289 WV = T; Ath, Mrg, Nob, Moe = U
WAY Wayne relation 911304 U
WIL Williams relation 911287 IN, MI = T; Ful, Hen, Def = U
WOO Wood relation 911313 Luc = Yt; Ott, San, Sen, Han, Hen = U Luc Luc
WYA Wyandot relation 911296 U

*Boundary Sources:

  • U: USGS data import
  • M: USGS topographic map
  • T: TIGER data import (state/municipality borders)
  • Y: Yahoo! or other acceptable aerial imagery
  • t: Implied from non-boundary TIGER data