This page should not be maintained by aging confused amateur me.
Modern Taiwan maps will only have the following coordinates, if any:
TWD67 examples:
If the above are off by about 828 meters then the map is TWD97.
If you mix up TWD67 and TWD97, your results could be off by a kilometer -- dangerous [Chinese].
So, these days, beside longitude and latitude, one must also mention "TWD67" or "TWD97" for the data point to be useful. TWD97 A responsible map publisher should print both grids...
2007/3 To sum it up, rescuers should check three places:
They report |
Our map is |
We should check |
|---|---|---|
| WGS84 | WGS84 | original X,Y |
| WGS84 | TWD67 | X-828m, Y+207m |
| TWD67 | WGS84 | X+828m, Y-207m |
| TWD67 | TWD67 | original X,Y |
Recommendation to rescuers: just like medical workers still will test the patient's blood type before giving them a transfusion even though they insist they are type AB etc., we shouldn't trust the reporting public's vague notions of datums, if any. Nor should we trust various rescue teams with piles or PDAs of various maps. Therefore the helicopter should check in the above three places, which form a west-northwest - east-southeast line. During the process the coordination center should not "help" anybody by converting the original reported coordinates, lest confusion increase exponentially. Note however that I have no rescue experience and the above is all a guess as to how things should operate.
| Latitude origin | 0 degrees |
| Central meridian | 121 degrees E |
| Scale factor | 0.9999 |
| False easting | 250000 m |
| False northing | 0 m |
They are both 2 degree wide transverse Mercator projections. They differ in their ellipse.
Go to the junction of the equator and 121 degrees east longitude, then point one's head west and go straight 250 km. When one stops one will be considerably above the earth's surface. I'm pretty sure that is the "origin".
Not exactly. Take a good look at a map. The only place where the longitude and latitude grid lines are parallel to the TM2 grid lines is 250 km E., i.e., 121 east longitude, that single line. All other lines are more or less askew.
proj is free software:
GNU/Linux example, Dongjundashan TWD67: given x and y, compute longitude and latitude (and store them in variable
LL):$ LL=$(echo 258566.571 2613894.788 | proj -I +proj=tmerc +ellps=aust_SA +lon_0=121 +x_0=250000 +k=0.9999) $ echo $LL 121d5'2.255"E 23d37'42.655"NNow convert back to x and y:
$ echo $LL | proj +proj=tmerc +ellps=aust_SA +lon_0=121 +x_0=250000 +k=0.9999 258566.57 2613894.80(If we were using TWD97 instead of TWD67, we would instead use:)
$ proj +proj=tmerc +ellps=GRS80 +lon_0=121 +x_0=250000 +k=0.9999The ellipses we use can be seen with "
proj -le":
For TWD97 use :
GRS80 a=6378137.0 rf=298.257222101 GRS 1980(IUGG 1980)For TWD67 :
- Use:
aust_SA a=6378160.0 rf=298.25 Australian Natl & S.Amer. 1969- DON'T use:
GRS67 a=6378160.0 rf=298.2471674270 GRS 67 (IUGG 1967)I.e. the TWD67 spheroid is NOT the International 1967. Names get confused; parameters are what count.
- Q: the HTZ (Hu Tzu Shan) datum predefined in ArcInfo/ArcGIS is set to Int/Hayford 1909. Have ESRI (Arc perpetrators) made a mistake?
A: Try the aust_SA parameters instead. The name "Hu-tzu-shan" is full of danger, as it has accrued several definitions. One must instead talk in terms of a=, f=.
/usr/share/proj/epsghas a# Hu Tzu Shan <4236> +proj=longlat +ellps=intl +towgs84=-637,-549,-203,0,0,0,0 +no_defs <>which is suspect too.
A: Well, since it is Free Software, perhaps see the source code to proj ? Also note freegis.org .
GPSFUN has a comparison like:
| Datum | Reference spheroid | major axis "a" | flattening "f" |
|---|---|---|---|
| see note * | see note * | 6378388 | 1/297 |
| TWD67 | GRS67 | 6378160 | 1/298.25 |
| TWD97 | GRS80 | 6378137 | 1/298.257222101 |
| WGS84 | WGS84 | 6378137 | 1/298.257223563 |
* note: the names "hu-tzu-shan", and even "International 19xx", have been given many definitions here in Taiwan, so are dangerous to use. One must speak in terms of a, f !
"hu-tzu-shan" sometimes even means TWD67.
Online TM to lat/long transformation (for TWD67 to TWD67, TWD97 to TWD97 will be off by .0001 degree), [Chinese] but no source code, no decimals below 0.0001 degrees, no clear website name.
TWD97 is very close to WGS84, at least where I live,
$ a="120d51'58.2\"E 24d10'54.2\"N"; echo $a #(just see what it looks like)120d51'58.2"E 24d10'54.2"N
$ echo $a | proj +proj=tmerc +ellps=GRS80 +lon_0=121 +x_0=250000 +k=0.9999 -f %.5f236402.76293 2675155.08596
$ echo $a | proj +proj=tmerc +ellps=WGS84 +lon_0=121 +x_0=250000 +k=0.9999 -f %.5f236402.76293 2675155.08604
$ echo 2675155.08604 - 2675155.08596 | bc.00008
gpsman author "M" == Miguel Filgueiras writes:
- M> As far as I understand from your description these 2 degree zones centered from 115 to 125 will cover longitudes from 114 to 126. Does these cover all Taiwan territory?
yes, everything that Taiwan owns or still claims to own.
- M> I also take it that these zones cover all the latitude range, that is, for each central meridian there is a single zone for all the possible latitudes in Taiwan.
Yes, and each has Y=0 at the equator, and X=250000 at the central meridian.
- M> Another point: could you please confirm that the correct scale factor is k0=0.9999.
yes, that's right, 0.99990 And note that these 6 zones can be used with both the TWD67 or TWD97 datums.
I discovered many maps of Jinmen , at least as late as the 1990's still use an older system:)
$ proj +proj=utm +ellps=intl +lon_0=117 # intl a=6378388.0 rf=297. International 1909 (Hayford)
(3/2008: Government's closed-source version: .PDFs, .ZIPs. Anyways, ignoring them we do it by hand:)
This is the best I can do at present, for all of Taiwan, with its few meters of error. (828: Odd, all the few formula I have below give approximately 841, why?)
Also john at thl.ncku.edu.tw 8/2003 contributed:
A = 0.00001549 B = 0.000006521 X67 = X97 - 807.8 - A * X97 - B * Y97 Y67 = Y97 + 248.6 - A * Y97 - B * X97 X97 = X67 + 807.8 + A * X67 + B * Y67 Y97 = Y67 - 248.6 + A * Y67 + B * X67 Perl implementation
2/2006: Now there is Perl Geography-NationalGrid-TW which perhaps solves many problems discussed by this page. 2007/10: xylonlat is an example program I wrote using it.
Minstrel made programs, 6/2004.
2007: note http://www.mobile01.com/topicdetail.php?f=130&t=374120
I'm not sure what the official Z axis transformation is for TWD67--TWD97.
There are various programs here in Taiwan to convert from TWD67 to TWD97, but their authors or organizations are not willing to release them. Also most are for Micro$oft. Therefore, I wish there would be someone who understands coordinate transformations to step forth and under the free software license write a TWD67 -- TWD97 transformation program and also put its source code on the web. Who knows, perhaps in the future everybody will use ours!
Tools might only be for Windows.
From a fortran program I was given, I guess proj's cs2cs parameters might be
$ cs2cs +proj=tmerc +ellps=aust_SA \ +towgs84=-764.558,-361.229,-178.374,-.0000011698,\ .0000018398,.0000009822,.00002329 +lon_0=121 +x_0=250000 \ +k=0.9999 +to +proj=tmerc +datum=WGS84 +lon_0=121 +x_0=250000 +k=0.9999
The results and transfer.for's are close : for the whole island around x: -841, y: 207, (z: varies a lot). Maybe someone who understands this could lend a hand.
Note that the above is a seven parameter transformation different than:
Chatting with M.H. Ho of Taiwan Garmin, in 2000 :
- Q: I need dx,dy,dz,da,df for Taiwan Hu-tzu-shan datum (note: "Hu-tzu-shan datum" is Garmin's name for TWD67 )
A: For GARMIN GPS:
- dx: -767 m
- dy: -358 m
- dz: -176
- da: -18.00
- df: -0.00081204
- Q: Garmin's GPS units, at least to the year 2000, have the flaw that when we select "Taiwan Grid", the unit displays "T67". Note that that should be related to the "Hu-tzu-shan datum" [Garmin's name for TWD67]. How could one show a "67" when one selects WGS84? [The "67" has nothing to do with the grid. We use the same grid (X shifted 250000m etc.) for both TWD67 and WGS84 datums.] Of course we can still use the unit, it just looks bad.
A: Thanks for your letter. Indeed when one switches to WGS84, still showing T67 can create confusion. I will report this to our company. Thanks.
Perhaps Garmin still hasn't fixed this. Not only does it look bad, but it also creates danger on the trail.
2003.6.26 Cygnet said: After bugging them about it so long, new Garmins now say "TM2".
So, new Garmin models display:
| Old/New | Grid | ||
|---|---|---|---|
| Lat,Lon | TM-2 | ||
| Datum | TWD67 | N,E/N,E | T67/TM2 |
| WGS84 | N,E/N,E | T67/TM2 | |
They no longer use the name of a datum for a grid. However one still can't tell what datum has been chosen from this display.
2004/3: Garmin products now have an additional "TAIWAN" datum now along with their original so-called "Hu-tzu-shan" datum. Apparently this new "TAIWAN" datum is supposed to be just a minor correction to their so-called "Hu-tzu-shan" datum. The instruction books apparently say this "TAIWAN" means TWD67, which would be apparently correct. (Why doesn't Garmin just name it directly "TWD67"?)
unofficial Garmin GPS cheap connector [Taiwan]
Regarding DA, DF, DX, DY, DZ (but beware of any 1924 based Hu-tzu-shan datums listed there).
If in a land office you stumble upon an old grid that uses 20/11 meters per unit, that is the so called Cadastral coordinates.
Programs: cad2twd67
$ echo 120.85788 24.18347|./longlat2taiwanmapno #1> 120.85788 24.18347 9521-2-025 9521-2-09 9521-2-NW $ echo 9521-2-025|./taiwanmapno2longlat #1> 9521-2-025 120.850 24.175
TWD67 Taiwan Triangulation Points' Coordinates
Orthophotomaps are labeled from 1-100 instead of the more systematic 0-99. See Chinese version.
Odd, on maps from the 1980's there is traces of a 1 km. diameter circular road centered at TWD67 213230 mE, 2676040 mN, (north of Fengjia Univ., Taizhong). Traces of an old cyclotron?
My friend guesses it might be a high frequency direction finder, like the ones in Linkou, TWD67: 2776.5 km N 289 km E, used in cooperation with the USA in coordination with others in Asia to triangulate signals from China. However the diameter of the largest Linkou one is only 1/3 km.
Why isn't Taizhong's Daya Interchange on Zhongqing Rd. but instead on an extra side road? Looks so curious on a map. Doesn't that just increase the number of red lights? See Chinese version.
The finder of 24N 121E reassures me he was using WGS84 and not TWD67...
Working with Coordinates and Units
bnpcoa2shp: Taiwan cadastral .coa, .bnp format conversion to Shapefile.
Last modified: 2008-03-26 06:47:24 +0800