I Total length of the NHs in India
In the Annual Report 2021 the total length is 136, 440 km. This is the total sum of the NHs in all states.
If we take the individual lengths of the NHs (values from 2019) : 131048 km.
But since 2019 some NHs are newly declared and these length are not published.
If we add the mapped OSM-values of those Nhs, we get 132,627km. 10 Nh lengths are still missing.
Here are the correct mapped lengths in all states without counting ways double.
The sum is 126,913km.
Some NHs are not mapped. If we add here the MORTH values, we get 128,185km.
For 11 NHs there are no lengths at all.
Counting all trunk-ways in OSM (declared and not declared) there are 129,991km
That means MORTH has at least 4442 km more than OSM (3.3%).
By the way: we have now 618 NHs in India!
II Declaration of the NHs
The NHs are declared with the notifications of MORTH, published at https://egazette.nic.in
Here all the route informations of the notifications are listed.
These route informations are problematic:
a) often the data is ambigous.
There are more than one possible way to pass from one town to the next. In these cases I made corrections according to the Bhuvan map.
As far as I know this is the only available map which uses the new NH-numbers. (MORTH does not have a detailed map)
So the OSM length get more similar to the MORTH-values.
b) The data gives no information about the route through big towns and cities.
c) Bypass constructions
In the last years a lot of Bypass-roads have been constructed. I mean that MORTH only counts the Bypass route, but there are no secure informations.
d) there are a lot of cases where NHs have been declared, but they are only planned or under construction
III Mapping of NHs in OSM
According to the wiki NHs are highway=trunk.
If the road conditions are bad, this should be mapped with other tags.
In very big cities mappers insist to downgrade NH-ways to primary/secondary. I did not change this. But these ways are not counted in the NH-length.
trunk_link : These ways conect a NH to other roads but are not members of the NH-relation.
All ways which you pass from beginning of a trunk to the end are trunks!
The bypass roads are members of the NH-relation.
highway=construction with construction=trunk are not members of the NH-relation
expressways /motorways are not trunks and are not counted
b) Who complete is the NH-mapping?
More than 99% of all NHs are mapped.
c) Precision of mapping
The most important NHs are very exactly mapped. (Thanks to all mappers!)
I suppose the unprecision of minor Nhs is not greater than 5%.
IV calculation of NH length
I used the following overpass queries:
for a single NH : http://overpass-turbo.eu/s/1b6l
for the length of all NHs ina state : http://overpass-turbo.eu/s/1b6m
These queries count all highway=trunk which are members of the NH-relation. That means the relations have to be complete. At the moment they are complete. But some mappers remap ways and forget to update the relation. Also the oneway=yes tag must be correct. Otherwise the dual carriageways are not counted correctly.
There are still some NH-ways which are mapped as trunk_link and so they are not counted. I was not able to find all these cases.
V precision of length calculations
Because overpass does not use elevation data, the NH-length in high mountain areas are to small (upto 10% I guess). I suppose MORTH does include elevation data.
VI Main reasons for the differences between MORTH and OSM lengths
a) trunk ways which are shared by two or more NHS :
In this case the NH-ways have two or more ref-values. So in the map all NHs are completely to be seen.
It seems Morth made decisions that each way has only one ref-value and these ways count only in the length of this single NH.
This is the main reason for the difference values MORTH/OSM.
Because the NHs have been renumbered, the road signs are not always existing with the new numbers.
What is the actual situation about road signs of NH-numbers?
In OSM we have the „on the ground“ principle. If the signs say this is NH44 and not NH48 then we have to change the mapping and the relations. But at the moment I think that is not possible and MORTH does not give any informations to this problem.
b) elevation data
Since we do not have elevation data in OSM the error in mountain areas can be 5 to 10%.
I made a spreadsheet with all calculated lengths. If someone is interested, I can offer it for download.