The parcels are, of course, horizontal only datum. from the .prj PROJCS["NAD_1983_StatePlane_New_Jersey_FIPS_2900_Feet", GEOGCS["GCS_North_American_1983", DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]], PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]], PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",492125.0], PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-74.5], PARAMETER["Scale_Factor",0.9999], PARAMETER["Latitude_Of_Origin",38.83333333333334],UNIT["Foot_US",0.3048006096012192]]
and from a representative flood map .prj PROJCS["WGS_1984_Web_Mercator_Auxiliary_Sphere",GEOGCS["GCS_WGS_1984", DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]], PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]], PROJECTION["Mercator_Auxiliary_Sphere"],PARAMETER["False_Easting",0.0], PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",0.0], PARAMETER["Standard_Parallel_1",0.0],PARAMETER["Auxiliary_Sphere_Type",0.0],UNIT["Meter",1.0]] which seems now, looking at them, should just be a standard transform. But then this from Adv_MO_FloodZones.shp.xml -<geodetic> <horizdn>GCS WGS 1984</horizdn> <ellips>World Geodetic System 1984</ellips> <semiaxis>6378137.000000</semiaxis> <denflat>298.257222</denflat> </geodetic> </horizsys> <vertsys> <gridsysn>NAVD88</gridsysn> <plandu>FEET</plandu> </vertsys> well, I'll continue to muddle and head scratch. ---------------------------------------- > From: g...@ir.bbn.com > To: sgl...@hotmail.com > CC: gdal-dev@lists.osgeo.org > Subject: Re: [gdal-dev] Fema Flood Maps NAV88 - proj4text > Date: Sat, 2 Feb 2013 08:21:11 -0500 > > > You may want to join the proj list. > > Your parcel maps are in EPSG (ESRI, really) 102711, which is: > > http://spatialreference.org/ref/esri/102711/ > > This is a horizontal-only datum. I have only encountered horizontal > data in parcel maps (Mass, not NJ). Do you have any heights presented > with this? If so, do you understand what they are? If there are > heights, I would expect they are in NAVD88, but they could be in > ellipsoidal height relative to the GRS80 ellipsoid and the NAD83 > orientation. Ellipsoidal height is relatively recent, and estimated > orthometric heights based on geoid models and observed ellispoidal > heights is where NGS is heading, but I would doubt most practitioners > are doing that yet. > > Your flood maps are in EPSG:5703: > http://spatialreference.org/ref/epsg/5703/ > This is a vertical-only datum. Are there horizontal coordinates? If > so, what are they in? I would expect some sort of NAD83, and their > purpose is to locate where the heights are given. > > So what I think you want is to know the NAVD88 heights around the parcel > of interest, so you therefore want all your horizontal coordinates in > NAD83, and to leave the height in NAVD88. (Water flows downhill in > NAVD88, but does not necessarily flow towards lower height values in > ellipsoidal height coordinates!!) > > You were talking about a geoid file, which presumably transforms NAVD88 > height to something, which is probably ellipsoidal height based on > NAD83. That's the typical geoid model published by NGS. > There are also geoid models from WGS84 ellipsoidal height, but these do > not transform to NAVD88. > > Do you have any height data in other than NAVD88? If so, please > describe it. _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
