A suggestion for improving the accuracy of LMT of Asia/Tokyo
To whom it may concern, First of all, I'd like to express my appreciation for your maintaining the time zone database over the world. With regards to the current LMT of Asia/Tokyo, there are two problems. 1. It is based on 'Tokyo Datum' using Bessel ellipsoid of which equatorial radius, and flattening are less accurate than GRS80. 2. Location of origin point used by 'Rika Nenpyou (until 2009)' was not commonly used for geodetic reference system in Japan. To fix problem #1, which is illustrated in detail below as (Zt) and (Zj), E 139 44' 40.90" (9h 18m 58.7266s) should be E 139 44' 29.27" (9h 18m 57.9513s) To fix problem #2, which is illustrated in detail below as (Zj) and (Xj), E 139 44' 29.27" (9h 18m 57.9513s) should be E 139 44' 28.8759" (9h 18m 57.9250s) The influence of problem #2 is insignificant, but problem #1 causes a one-second difference. Therefore, though the priority of pursuing accuracy of LMT may be low, I believe that there is not much reason in continuing to use the current value, and the line should be updated as: --- tzdata2010o/asia 2010-11-01 22:18:22.000000000 +0900 +++ tzdata/asia 2011-01-10 13:25:42.000000000 +0900 @@ -1280,7 +1280,7 @@ # ordinances took effect on Jan 1. # Zone NAME GMTOFF RULES FORMAT [UNTIL] -Zone Asia/Tokyo 9:18:59 - LMT 1887 Dec 31 15:00u +Zone Asia/Tokyo 9:18:58 - LMT 1887 Dec 31 15:00u 9:00 - JST 1896 9:00 - CJT 1938 9:00 Japan J%sT --------------- The reasons are as follows: * Geodetic reference system in Japan[*1][*2JA]: 1918(?)-2002/3/31: Tokyo Datum(Bessel) 2002/4/1- : Japanese Geodetic Datum 2000 - JGD2000(ITRF94,GRS80) * Location of origin point(normally used for geodetic reference system in Japan)[*1 4.4.2][*8JA]: (X')1892-1923/9/1 "The center point of the Merz-Repsold meridian circle of the former Tokyo Astronomical Observatory, Azabudai 2-18-1, Minato-ku, Tokyo" (X)1923/?/?- "The intersection point of the cross carved on the metal mark of Nihon Keiido Genten, Azabudai 2-18-1, Minato-ku, Tokyo" * The base point of "Tokyo" in Rika Nenpyou[*5JA][*6JA][*7JA][*8JA]: (Z)1925-2009 "The site of the center point of the Repsold transit instrument of the former Tokyo Astronomical Observatory, Azabudai 2-18-1, Minato-ku, Tokyo" (Z) was located at 10meter east of (X')[*4JA]. 2010- They changed the point to same as (X). ** Those points in Tokyo Datum(Bessel) (Xt): E 139 44' 40.5020" N 35 39' 17.5148" [*5JA] => 9h 18m 58.7001s (Zt): E 139 44' 40.90" N 35 39' 16"(*) [*5JA] => 9h 18m 58.7266s (Zt) is the value taken currently by the TZ database(2010o). (*) Since longitude and latitude of (Zt) are based on the different measurements[*5JA], and (Z) was located at 10meter east of (X')[*4JA], (Zt) should be about E 139 44' 40.90" N 35 39' 17.51". The value of (Zt) above is not normal value, but its effect on the symptom is slight, since the difference is in the latitude. ** Those points in JGD2000(=~ WGS84) (Xj): E 139 44' 28.8759" N 35 39' 29.1572" [*10][*11][*3JA][*5JA][*6JA][*7JA] => 9h 18m 57.9250s (Zj): E 139 44' 29.27" N 35 39' 27.7" [*5JA] => 9h 18m 57.9513s Additional Note: Solar longitude: As for solar longitude, threre is data that Akashi Municipal Planetarium is located on E 135(9h) in solar longitude[*9JA]. E 135 in "Tokyo Datum" is 370meter west of the planetarium[*9JA]. E 135 in "WGS" is 120meter west of the planetarium[*9JA]. 120meter east-west is about 4.712"(0.314s) in WGS longitude at the point. 9h 18m 57.9250s(Xj) + 0.314s = 9h 18m 58.24s "9:18:58" seems to work even concerning for the solar longitude, although 0.314s is not a value at (X). - References Geospatial Information Authority of Japan http://www.gsi.go.jp/ENGLISH/index.html National Astronomical Observatory of Japan http://www.nao.ac.jp/E/index.html [*1] Concept of the New Japanese Geodetic System http://www.gsi.go.jp/common/000001195.pdf [*2JA] http://www.gsi.go.jp/LAW/G2000-g2000faq-1.htm (in Japanese) [*3JA] Nihon Keiido Genten (Origin point of Japanese Geodetic System) http://www.gsi.go.jp/kanto/ki8cgenten.html (in Japanese) [*4JA] http://www.gsi.go.jp/kohokocho/kohokocho40182.html (in Japanese) [*5JA] Rika Nenpyou (Chronological Scientific Table) 2010 p.Koyomi76(76) 2009/11/26 National Astronomical Observatory of Japan http://www.rikanenpyo.jp/ (in Japanese) [*6JA] Rika Nenpyou (Chronological Scientific Table) 2010 p.4 [*7JA] Rika Nenpyou (Chronological Scientific Table) 2011 p.4 2010/11/26 National Astronomical Observatory of Japan [*8JA] http://www.gsi.go.jp/kohokocho/koho507-2.html (in Japanese) [*9JA] Akashi Municipal Planetarium http://www.am12.jp/faq/shigosen.html (in Japanese) [*10] Realization of Horizontal Geodetic Coordinates 2000 2.3.3 http://www.gsi.go.jp/common/000001196.pdf [*11] RealizationGeographical Survey Institute Japan 2005 p.10 http://www.gsi.go.jp/ENGLISH/ABOUT/REPORT/Annual_2005.pdf - --- I hope this information helps. Thank you and best regards, --- Fumiyuki Shimizu 清水文行 fumifumi@abacustech.jp Abacus Technologies, Inc.
On Wed, 12 Jan 2011, Fumiyuki Shimizu wrote:
With regards to the current LMT of Asia/Tokyo, there are two problems. 1. It is based on 'Tokyo Datum' using Bessel ellipsoid of which equatorial radius, and flattening are less accurate than GRS80. 2. Location of origin point used by 'Rika Nenpyou (until 2009)' was not commonly used for geodetic reference system in Japan.
That line in the rules ...
# Zone NAME GMTOFF RULES FORMAT [UNTIL] Zone Asia/Tokyo 9:18:59 - LMT 1887 Dec 31 15:00u
... is talking about the local mean time as it was before 31 Dec 1887; it's not the "current" LMT. The oldest reference in your message seemed to be for the year 1892. Do you have information about the official time in Tokyo before or during the year 1887? --apb (Alan Barrett)
Hello, Thank you for the message.
Regarding Re: A suggestion for improving the accuracy of LMT of Asia/Tokyo; Alan Barrett <apb@cequrux.com> adds:
With regards to the current LMT of Asia/Tokyo, there are two problems. 1. It is based on 'Tokyo Datum' using Bessel ellipsoid of which equatorial radius, and flattening are less accurate than GRS80. 2. Location of origin point used by 'Rika Nenpyou (until 2009)' was not commonly used for geodetic reference system in Japan.
That line in the rules ...
# Zone NAME GMTOFF RULES FORMAT [UNTIL] Zone Asia/Tokyo 9:18:59 - LMT 1887 Dec 31 15:00u
... is talking about the local mean time as it was before 31 Dec 1887; it's not the "current" LMT.
I see. This leads me to believe that there is one more issue of LMT of Asia/Tokyo in the current(2010o) tz database: 3. The value(9:18:59) of LMT of Asia/Tokyo, which is for time before 1888-01-01 00:00, is based on the location of an instrument of the Tokyo Astronomical Observatory, which was established in 1888[*12]. [*12] http://www.nao.ac.jp/E/about/history.html
The oldest reference in your message seemed to be for the year 1892. Do you have information about the official time in Tokyo before or during the year 1887?
No, I don't, but it seems correct that the known official time in Tokyo has started at 1888-01-01 00:00, and lines of Asia/Tokyo seems correct except for the value 9:18:59. In the tz database, I believe LMTs are not the official time, but are mostly the solar time based on the longitude of the typical place, measured with the current technology, such as GPS, to substitute for the culmination time of the Sun in the equation of time, one of the reasons for which may be to fill the absence of the official time. Is my assumption correct? It is with this assumption that I wrote in my previous mail that the priority of pursuing accuracy of LMT might be low. Please let me revise my explanation about LMT and the definition of "Tokyo" based on the information you've given me. The value(9:18:59) of LMT of Asia/Tokyo in tz database(2010o) is based on 'Rika Nenpyo 1996' as described in the file. 'Rika Nenpyo 1996' set the site of the center point of the Repsold transit instrument of the former Tokyo Astronomical Observatory as 'Tokyo,' and displayed the location in Tokyo Datum. Tokyo Datum is not suitable for a solar time *even in the past*. In other words, the value(9:18:59) is neither the official time nor accurate as the solar time because it is expressed in Tokyo Datum, the obsolete geodetic reference system in Japan(#1). Most LMTs in the tz database may be based on locations expressed in WGS84 and its variant, but the value(9:18:59) of Asia/Tokyo is in Tokyo Datum, which is 9:18:58 in WGS84 as described in the previous mail(#1). On the other hand, the Tokyo Astronomical Observatory was established in 1888. If this raises a question 'What is Tokyo?' about current(2010o) LMT of Asia/Tokyo which is for time before 1888-01-01 00:00, the following information might help: Edo was renamed to Tokyo on September 3rd, 1868, by "Shousho"(Emperor expressed himself on official business). [*13] [*13] http://ja.wikisource.org/wiki/%E6%B1%9F%E6%88%B8%E3%83%B2%E7%A8%B1%E3%82%B7%... (in old Japanese) It seems that the symbol of Tokyo in 1868 was Edo-jo castle[*14]. To fix problem #3 by using the location of Edo-jo castle, E 139 45' 15.72" [*15]([*16]), => 9h 19m 01s [*14] http://en.wikipedia.org/wiki/Edo_Castle [*15] http://ja.wikipedia.org/wiki/%E6%B1%9F%E6%88%B8%E5%9F%8E (in Japanese) [*16] http://toolserver.org/~geohack/en/35_41_17.97_N_139_45_15.72_E_scale:20000?p... --- tzdata2010o/asia 2010-11-01 22:18:22.000000000 +0900 +++ tzdata/asia 2011-01-22 22:07:54.000000000 +0900 @@ -1280,7 +1280,7 @@ # ordinances took effect on Jan 1. # Zone NAME GMTOFF RULES FORMAT [UNTIL] -Zone Asia/Tokyo 9:18:59 - LMT 1887 Dec 31 15:00u +Zone Asia/Tokyo 9:19:01 - LMT 1887 Dec 31 15:00u 9:00 - JST 1896 9:00 - CJT 1938 9:00 Japan J%sT ---------- As for the time before September 3rd, 1868, it is uncertain as to how it should be expressed, because: 1868 Edo was renamed to 'Tokyo.' (no official time) 1457 Edo castle was built. (no official time) ... I hope this information helps. P.S. 'AT' fields in lines starting with 'Rule Japan' seems to be wrong(it was 0:00, not 2:00) and I have ordered the book written by 'Shanks & Pottenger,' which is referenced by those values to search the root cause of the difference. I might post a patch in the future. Thank you and best regards, --- Fumiyuki Shimizu Abacus Technologies, Inc.
participants (3)
-
Alan Barrett -
Fumiyuki Shimizu -
Fumiyuki Shimizu 清水文行