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[GMT] 使用GMT软件绘制SRTM地图

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发表于 2013-2-23 10:35:07 | 显示全部楼层 |阅读模式
SRTM3 DEM数据的分辨率是3″,约为90米,美国境内公开的还有SRTM1,地面分辨率为30米。SRT M3 的标称绝对高程精度是16 m, 标称绝对平面精度是20m。而ETOPO5的分辨率是5′,较新的ETOPO1分辨率是1′,GTOPO30的分辨率是30″,其分辨率和精度都不如SRTM。

本文使用HGT后缀的SRTM3数据,给出GMT绘制其SRTM的方法。
HGT后缀的SRTM数据下载地址 :  http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/

SRTM被分割成1°×1°的方格,文件命名和经纬度有关,如N37W105 表示左下角为37N,105E的1°×1°方格。数据文件后缀为.HGT。

其他一些SRTM基础信息:
QuickStart
Welcome. If you just can't wait to start playing with the SRTM data, or
just don't have the patience for reading tedious documentation, you'll need
to know at least the following. For more detail see SRTM_Topo.doc.
SRTM data are distributed in two levels: SRTM1 (for the U.S. and its territories
and possessions) with data sampled at one arc-second intervals in latitude and
longitude, and SRTM3 (for the world) sampled at three arc-seconds. Three
arc-second data are generated by three by three averaging of the one
arc-second samples.
Data are divided into one by one degree latitude and longitude tiles in
"geographic" projection, which is to say a raster presentation with equal
intervals of latitude and longitude in no projection at all but easy to manipulate
and mosaic.
File names refer to the latitude and longitude of the lower left corner of
the tile - e.g. N37W105 has its lower left corner at 37 degrees north
latitude and 105 degrees west longitude. To be more exact, these
coordinates refer to the geometric center of the lower left pixel, which in
the case of SRTM3 data will be about 90 meters in extent.
Height files have the extension .HGT and are signed two byte integers. The
bytes are in Motorola "big-endian" order with the most significant byte first,
directly readable by systems such as Sun SPARC, Silicon Graphics and Macintosh
computers using Power PC processors. DEC Alpha, most PCs and Macintosh
computers built after 2006 use Intel ("little-endian") order so some byte-swapping
may be necessary. Heights are in meters referenced to the WGS84/EGM96 geoid.
Data voids are assigned the value -32768.
SRTM3 files contain 1201 lines and 1201 samples. The rows at the north
and south edges as well as the columns at the east and west edges of each
cell overlap and are identical to the edge rows and columns in the adjacent
cell. SRTM1 files contain 3601 lines and 3601 samples, with similar overlap.

例子首先选择起伏较大的昆仑山区域,下载的数据为N37E085.hgt和N37E086.hgt 2个文件。

Rem 首先使用xyz2grd将二进制的hgt文件转成GMT使用的grd文件,或者nc后缀也可。
xyz2grd N37E085.hgt -Gg001.grd -I3c -R85/86/37/38 -N-32768 -ZTLhw
xyz2grd N37E086.hgt -Gg002.grd -I3c -R86/87/37/38 -N-32768 -ZTLhw
Rem 合并grd文件
grdpaste g001.grd g002.grd -Ga1.grd
makecpt -Cglobe -T-500/7000/500 -Z > mydata.cpt
grdgradient a1.grd -A30 -Gchina.grd.int -Ne0.8 -V -M
Rem 使用grdimage绘制彩图
grdimage a1.grd -R85/87/37/38 -Yc -Xc -JM10i -Cmydata.cpt  -Ichina.grd.int -K > world1.ps
pscoast -R -J -Di -N1 -A1000 -B0.5g0.5/0.5g0.5 -W0.25p  -O -K  >>world1.ps
psscale -Ba2500f500::/:"m": -Cmydata.cpt -D12.5c/-2c/15c/.35ch -O -U"author:YangLei">> world1.ps
del *.cpt *.int
结果如下图所示:

1.jpg


由于SRTM3数据在海洋是海面高,而ETOPO1的海洋数据含有海底地形,二者有较大区别。为此在太平洋中选择一处岛礁区域做比较。下面是其对比,可见SRMT3的分辨率要明显的高处ETOPO1,另外ETOPO的海底地形信息是SRTM数据所不具有的。因此绘制地图时,要结合实际要求,选择高程数据。
2.jpg 3.jpg


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发表于 2013-3-10 22:46:26 | 显示全部楼层
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