A suggestion for improving the accuracy of LMT of Asia/Tokyo

Wed Jan 12 03:06:18 UTC 2011

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 at abacustech.jp
Abacus Technologies, Inc.

