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发表于 2006-5-28 12:17:07
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老虎，我猜那是因为GPS waypoint及topo map的格式问题,我们一般都用的是WGS84 Lat-Long(即Lat-Long, Degree minute, datum is WGS84)，但网上找来的waypoint可能会是不同的，比如
DD-WGS84 Lat-Long, Decimal Degree, datum is WGS84 DMS-WGS84 Lat-long, Degree minute second, WGS84 UTM-WGS84 Full UTM code, datum is WGS84 UTM-NAD27C Full UTM code, datum is NAD27Canada
再比如,在BC Basemap上用DMS-WGS84重定位waypoint方便，即 Lat-long, Degree minute second.
这样的结果就自然会有很大的误差。
这些格式的waypoint都可以相互转换的，网上还有现成的Coordinate Converter软件可利用。
转贴一文供参考：
Map Coordinate Systems
This document explains coordinate systems such as UTM and Lat-Long, and also map datums such as WGS84 or NAD27Canada. Your position on the earth can be described in two different coordinate systems: Lat-Long and UTM. Both depend on which "map datum" you are using.
"Lat-Long" means latitude-Longitude. For example, 50 degrees and 29 minutes north, 123 degrees and 50 minutes west. Latitude-Longitude coordinates are sometimes referred to as the "geodetic grid" which we abbreviate to "Geo". "UTM" means "Universal Transverse Mercator". Canadian Topographic maps have both scales, but only have grid lines for the UTM scale. The Lat-Long system is simpler and more universal, and is what is always used behind the scenes in computer systems.
A set of coordinates alone does not specify your exact position, because you need to know what "map datum" applies to those coordinates. For example, if someone tells you that the US-Canada border is at 49 degrees north, you have to ask what datum they are using. 49 degrees by what datum? See Border Slash - Is it really the 49th parallel.
There is about a 200m difference, depending on the datum. The two main datums in Canada are WGS84 and NAD27. "NAD27" means "North American Datum 1927". "WGS 84" means "World Geodetic System 1984". Eventually all maps will be WGS84, because it is based on satelite measurements. However, most Canadian topographic maps are still NAD27. The difference can be as much as 200 meters, depending on your location. Here are some comparisons of the different co-ordinates for the same point: (Mt. Garibaldi)
WGS84 Geo coordinates: 49.850-123.008 NAD27 Geo coordinates: 49.850-123.0066 WGS84 UTM coordinates: 10 U 499425 5521953
GPS receivers can be configured to display your true position in a variety of position formats and datums. For example, you can set your GPS to read out a given waypoint in WGS84 UTM coordinates. Then you can change the readout to NAD27 Canada, and you'll see different coordinates for the same point.
The Bivouac website has a similar capeability. You can display the waypoints in any one of several common combinations. The default is WGS84 Lat-Long. But if you want to mark those points onto a typical canadian topo map, then you should click the "UTM-NAD27C" link. This will redisplay the waypoints as UTM, and using the NAD 27 Canada datum.
Now that you know the general idea of lat-longs, UTM, and datums, we can now look at the different notations for writing these things down.
Lat Long Notations:
Ideally, latitudes and longitudes would be given as decimal numbers, like any other number. For example: 50.778 degrees North. But for historical reasons, most paper maps have their scales in degrees and minutes. A minute is 1/60 of a degree. So we have to accommodate that format and translate it to decimal behind the scenes.
Given that we have to input values in degree-minute format, the first problem is that there is no degree symbol on the keyboard. Although we can display lat-longs using degree symbols (Eg: 50 ° 27.2') you can't directly type such a thing, so we use a colon for the separator between degrees and minutes. For example:
50:27.5
Note that in the above, there is only one colon, and the fractional part of a minute is delimited by a decimal point. That format is generally referred to as "Degree-Minute" (DM) format. The other common format you see in a lot of digital mapping packages is Degree-Minute-Second format (DMS format). In this format, the previous lat long becomes:
50:27:30
Note that the .5 minutes becomes 30 seconds.
The next requirement is a standard for writing both coordinates. We always put the latitude first, followed by the longitude. This has generally been the standard for the past 3 centuries, although lately you will see systems like BC Basemap which have it reversed.
Given that the latitude will always be first, and that all the values we are dealing with are North and West, we don't need to put "N" and "W" into our coordinates. All of North America is north and west. Therefore we just write our lat-longs as two strings separated by a dash. Anywhere in Bivouac that you are supposed to input a lat-long, they are combined. Below is the position of Mount Garibaldi in DM format:
49:51.0-123:00.5
We will accept lat-longs in any of the three formats: decimal degree, degree-minute, and degree-minute-second format. Which format is used usually depends on the source of the data: anything coming from BC Basemap will be in DMS format, and most points from paper maps will be DM format.
Note that our format is only good for North America. Most worldwide computer systems use plus and minus to indicate North versus South, and east versus west. In this case, you would have to separate the lat and long with a space, as shown below:
49:51.0 -123:00.5
UTM Formats
Before computers, the traditional method used by Canadian mountaineers was to communicate positions by using friendly looking grid references like "234-234 on map 92 G/15. The reason it looks simple is because nobody bothered to specify the datum, and they also threw away the upper digits, because they were giving the mapsheet. However, such simple minded grid references don't work once you get away from paper maps. Nowdays, if you are going to use UTM coordinates, you should understand the full UTM system. (Or use Lat-long).
The full blown UTM format has four parts as shown below:
10 U 499425 5521953
The first is the "Zone number", then the "Zone Letter", then the "Easting" and then the "Northing". Note that for some reason the UTM gives the east-west coordinate first, rather than following the lat-long standard of giving the north-south coordinate first.
The last decimal place is accurate to exactly 1 meter. Because a full UTM code is cumbersome, the grid lines on 1:50,000 maps are labelled by rounding off the last three digits, and also throwing away the first digits, such that only 2 digits remain. In the corner of the map the full number is given, but then each subsequent line is labelled with a two digit number, as shown below:
64 465000m 66 67 68 69 70 71 72 73 74
The above grid lines are 1 km apart. In mountaineering guidebooks, grid references are usually written to the nearest 100m which is about as accurate as you can estimate from a map. For example, an article might state that the whole trip was on map 92G/6, and then give grid references like GR 878-569 or GR 878569. |
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