QGIS等其他软件使用GDAL，会在安装程序中有个GDAL数据目录，里面包含了一些坐标系投影的数据。
我们自定义的投影坐标系没有EPSG代码，但软件有时候需要固定格式指定EPSG代码（例如定义GDAL的WMS地图服务的xml文件中需要定义EPSG代码）。如何在GDAL数据目录添加一个自定义的投影坐标系，给指定一个伪的EPSG代码？
下面是GDAL数据目录下esri_extra.wkt文件的内容。按照文件开头的意思，就是用WKT自己定义了几个伪的EPSG代码：31491，31492，31493等，这些代码不是EPSG机构真正指定的。
我自己在esri_extra.wkt文件中添加了一个EPSG代码和对应的wkt，但在QGIS中启动后，调用WMS地图服务的xml文件的EPSG代码失败了，提示GDALWMS:Bad projection specified。
是不是还要做其他设置呢？
This file derived from a subset of definitions from the "epsg" directory
distributed with ArcGIS. This file only contains definitions that aren't
real EPSG definitions. The definitions were "morphed" into OGR WKT format,
and the authority clause added during processing.
31491,PROJCS["Germany_Zone_1",GEOGCS["GCS_Deutsches_Hauptdreiecksnetz",DATUM["D_Deutsches_Hauptdreiecksnetz",SPHEROID["Bessel_1841",6377397.155,299.1528128]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",1500000.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",3.0],PARAMETER["Scale_Factor",1.0],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0],AUTHORITY["EPSG","31491"]]
31492,PROJCS["Germany_Zone_2",GEOGCS["GCS_Deutsches_Hauptdreiecksnetz",DATUM["D_Deutsches_Hauptdreiecksnetz",SPHEROID["Bessel_1841",6377397.155,299.1528128]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",2500000.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",6.0],PARAMETER["Scale_Factor",1.0],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0],AUTHORITY["EPSG","31492"]]
31493,PROJCS["Germany_Zone_3",GEOGCS["GCS_Deutsches_Hauptdreiecksnetz",DATUM["D_Deutsches_Hauptdreiecksnetz",SPHEROID["Bessel_1841",6377397.155,299.1528128]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",3500000.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",9.0],PARAMETER["Scale_Factor",1.0],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0],AUTHORITY["EPSG","31493"]]
