GEODAS Help for Design-a-Grid Downloaded GridsU.S. National Geophysical Data Center / World Data Center A for MGG
GEODAS Help for Design-a-Grid Downloaded Grids
ArcGIS/ArcView Grids
Design-a-Grid
Files
Formats
GEODAS
GEODAS Gridded Databases
Horizontal and Vertical Datums
The grids you downloaded from GEODAS On-line is the data found by you when you ran
GEODAS On-line Design-a-Grid. The data has been collected together and
compressed into a single base folder. Once uncompressed, You will find this base folder [Grid Id]_data with
several files, and a folder named for your Grid Id. The grid file(s) are in folder named [Grid Id].
Use this help file to understand the data formats and
other options you chose when creating the grid(s) on-line. Your grids will have been
extracted from one of several GEODAS Gridded Databases:
ETOPO2 Global Bathymetry,
US Coastal Relief Model and
Great Lakes Bathymetry.
All grid cell values are in meters.
You can use GEODAS On-line Download Software to access your data. Use Hydro-Plot for screenplots of the data. GEODAS Grid-Translator app can be used to re-grid your download grid files, changing lat/lon bounds, output format, etc. If you also downloaded the GEODAS Coastlines, you can use Coastline Extractor to view coasts and output to file. If you did not include the software with your data, and do not have it installed on your computer from earlier downloads or GEODAS DVDs, or if you wish to find the latest version, go to http://www.ngdc.noaa.gov/mgg/gdas/gx_announce.Html.
GEODAS (GEOphysical DAta System) is an interactive database management system developed by the National Geophysical Data Center (NGDC) for use in the assimilation, storage and retrieval of geophysical data. The GEODAS software is presently being used with several types of data: Marine Trackline Geophysical Survey Data, Hydrographic (bathymetric) Survey Data, Aeromagnetic Survey Data and GEODAS Gridded Data including ETOPO2 Global Bathymetry, US Coastal Relief Model and Great Lakes Bathymetry. For survey-based data searches by geographic area, year of survey, source, platform/project, survey identifier, or data type are available. Each survey is comprised of one Header record and multiple Data records. The Header record documents both the content and the structure of the subsequent Data records, containing that part of the data that remains invariant throughout the survey. The Data records present geophysical data with corresponding times and positions. Documentation that varies within the survey is also included with the data records. GEODAS Gridded Databases come with software with which the user can create custom grids. Other software includes Hydro-Plot data plot viewing, Re-Format application for changing formats (including xyz), and Coastline Extractor for viewing and saving coastline files.
GEODAS DVD sets come with access software for Microsoft Windows, Macintosh OS-X and UNIX Xwindows systems. The latest version of the desktop software can be found at www.ngdc.noaa.gov/mgg/gdas/gx_announce.Html Or this On-line version can be used for searching and downloading data on-line for free, creating custom bathymetry grids for free, or creating your own CD of data for purchase. GEODAS Desktop Software can also access downloaded data and grids, though with some limits to functionality for this data.
Contact the National Geophysical Data Center for more information. You can now use the NOAA National Data Centers Online Store for direct ordering of NGDC data. For more information about GEODAS products you can contact Robin.R.Warnken@noaa.gov For technical assistance contact Dan.R.Metzger@noaa.gov or John.G.Campagnoli@noaa.gov..
The data in GEODAS databases are of the best quality NGDC can obtain. Most have gone through extensive processing at their place of origin. The documentation headers that are included provide information on the accuracy of the data. NGDC makes no warranty as to the correctness of data values, but they are representative of the data as of their transfer to NGDC.
GEODAS Grid Translator - Design-a-Grid application can be run on-line to create a custom grid for several GEODAS Gridded Databases. These include: ETOPO2 Global Bathymetry, US Coastal Relief Model and Great Lakes Bathymetry. Users choose the criteria for their custom grid, including lat/lon area, grid cell size, format, number type, etc. The number of latitude and longitude cells of your grid will automatically be calculated and displayed at the bottom of the Design-a-Grid web page. When you are satisfied with your options click Design-a-Grid button to create the grid.
Grid Id - The Grid Id is a user-defined tag identifying the custom grid you create with GEODAS Grid Translator Design-a-Grid. It can be any combination of up to 8 alphanumeric characters plus "-" and "_" (no embedded spaces). This Id becomes the file prefix of the resulting grid file (suffix depends upon grid format).
Grid Database - You must choose a GEODAS Gridded Database. This will be the source of the grid cells for your custom grid:
Grid Lat/Lon Area Bounds - You must choose the latitude/longitude boundaries of your custom grid in degrees and minutes. Precision is to the nearest minute. Latitudes must be between 90 degrees North and 90 degrees South. Longitude values must be between 180 degrees West and 180 degrees East. Your grid may not cross both 0 degrees and 180 degrees. If you wish your grid to span the entire globe longitudinally (only relevent for ETOPO2 Global Topography), you can choose bounds of 180W to 180E, or from 0E to 0E. Depending on your source Grid Database and your output Grid Cell Size, you may be restricted in your boundaries. For example, if you have an output Grid Cell Size of 2 minutes, you may not choose a lat/lon minutes value of an odd number (e.g. 3 minutes). Initial values for a particular source Grid Database are the outer bounds of that database. Extending your bounds beyond that will only result in the extra cells being filled with the Empty Grid Cell value (see Grid Cell Value Parameters).
Grid Cell Size - You may change the cell size of your custom grid from that of the source Grid Database. However, your cell size must be a mulitple of the source database cell size. Only acceptable cell sizes will be presented in the dialog. Be aware that cell size and area are related and certain combinations will not work. For example, if you chose a lat/lon bounds minutes value of an odd number (e.g. 3 minutes), you would not be able to have a cell size of 2 minutes.
Grid Cell Value Parameters - You may choose values for various grid cell parameters, depending upon your source Grid Database. Some combinations may cause problems; a given number type/precision combination (i.e. 2-byte integers and tenths of meters) may not have sufficient size to hold the topographic values. The default values will suffice if you are not sure. Only relevant parameters will be presented in the dialog:
Grid Format - Choose your output Grid Format and Format Parameters.
Binary Raster Format: Simply a continuous series of binary depth values. If you also choose the GRD98 Header, the header will be attached to the beginning of the output file and you will have the GEODAS Gridded Database Format GRD98, part of the GEODAS MGD-2000 Exchange Formats package. This format can later be opened in GEODAS Software Application Grid Translator and you can then create new custom grids with different lat/lon bounds, cell size, number type, etc. GRD98 grids can be viewed and edited in GEODAS Application Hydro-Plot. GRD98 grids can also be imported directly into GMT (Generic Mapping Tool) Version 4. If you choose the ASCII (Arc) Header, the header will be in a separate file with a file extention of .hdr. This combination (Binary Raster w/ Arc Header) can be opened in GEODAS Hydro-Plot. If you chose Floating Pt as your number type this combination can be imported directly into ArcGIS, and perhaps other GIS systems.
ASCII Raster Format: A row by row series of space-delimited ASCII depth values. Each row ends with an End-of-Line indicator. The EOL will be a Carriage-Return/Line-Feed (ASCII Decimal Character Codes 13 and 10) for MS Windows and a New-Line (ASCII Decimal Character Code 10) for UNIX and Macintosh. You cannot add a GRD98 Header to this format. If you add ASCII (Arc) Header, the header will be attached to the beginning of the output file, and this file can be imported directly into ArcGIS and perhaps other GIS systems, and can be viewed and edited in GEODAS Hydro-Plot.
XYZ (Delimited lon,lat,depth): Each grid value will be a separate record containing a lon, a lat and a depth, the records being terminated by a record delimiter (Carriage-Return/Line-Feed for Windows or New-Line for UNIX and Macintosh). The longitude, latitude and depth values are separated by field delimiters (see below). XYZ files can be viewed and edited in GEODAS Hydro-Plot and imported into various GIS systems. If you choose the ASCII (Arc) Header, the header will be in a separate file with a file extention of .hdr.
GRD98 Header: This is the header for creating binary raster GEODAS Gridded Database Format GRD98 grids
ASCII (ArcGIS) Header: An ASCII grid header in the format used by Arc Systems. When used with the ASCII Raster Grid Format, the header file is attached to the beginning of the grid file. When used with Binary Raster Format and XYZ Format the header will be in a separate file with a file extention of .hdr.
No Header: No grid header will be associated with the grid data file. This is most often used with the XYZ (Delimited lon,lat,depth) Format.
XYZ Delimiter: For grid files of Delimited lon,lat,depth format only, choose space, tab or comma as the delimter to be inserted between the fields.
Omit Empty Grid Cells: For grid files of XYZ (Delimited lon,lat,depth) format you may include or omit the empty grid cell values in your output file. The default is to omit these values, which makes for easier importing into other applications.
Land/Sea Data Coverage - For US Coastal Relief Model only, Choose to transfer All Cells, Sea Cells Only, or Land Cells Only from the source Coastal Relief Model database. Un-transferred cells will contain the Empty Grid Cell Value.
Vertical Datum for Water Cells - For Great Lakes Bathymetry only, this option allows the use of local Great Lakes vertical datums for non-land cells. When using the default Mean Sea Level, water cells can have "depths" which are positive numbers. Using the Local Lake Datums will create grids with the proper water depths, but the land cells will still be based on acutal elevations (above mean sea level), thus showing a "cliff" where land starts.
For the NGDC Coastal Relief Model, virtually all the original elevation data that went into building the grids came from National Ocean Survey (NOS) surveys, which are based on low water vertical (sounding) datums, either Mean Low Water (MLW) or Mean Lower Low Water (MLLW). These raw data were not transformed to a common vertical datum before gridding, and the Coastal Relief Model reflects that data as is. The land topography data that went into the Coastal Relief Model were referenced to NAVD88.
Thus there is no single vertical datum for the Coastal Relief Model grids. Each cell's elevation value represents an average of elevations for the entire cell, which is 3 arc-seconds (~90 meters) by 3 arc-seconds in size, and at this scale the differences between vertical datums, typically less than 1 meter, are insignificant (averaging diverse elevations over a wide area will overwhelm any short vertical offset between different datums).
For all but Volumes 1 and 2, data used to create the Coastal Relief Model were transformed to NAD83 geographic coordinates horizontally. NAD83 is essentially identical to WGS84 geographic (the horizontal differences between the two are less than one meter). Data that have NAD83 or WGS84 horizontal datums can be merged without problem. The older NAD27 geographic datum is offset significantly, in many areas, from NAD83/WGS84 (up to a couple of hundred meters). Data in this datum should be transformed to NAD83/WGS84 when merging to Coastal Relief Model (or vice versa). For Volumes 1 and 2 (US East Coast), the data was not transformed to NAD83 prior to gridding, and thus represents various datums, mostly NAD27 and NAD83, but sometimes older datums. This will not present as much of an issue for these 2 volumes as it would have for some of the other volumes (based on undersea topography).
For a global relief model, like ETOPO2v2, which has 2 arc-minute (~4 km) cell size, the differences between vertical datums are considered to be not significant, so long as they are all near mean sea level (MSL). The Horizontal Datum of ETOPO2v2 is WGS84.
For Great Lakes Bathymetry grids, the Horizontal Datum should be considered as NAD83; the vertical datum of the water cells is either Mean Sea Level or Local Great Lakes Datums, depending on the option chosen (advanced). When using the default Mean Sea Level, water cells can have "depths" which are positive numbers. Using the Local Lake Datums will create grids with the proper water depths, but the land cells will still be based on acutal elevations (above mean sea level), thus showing a "cliff" where land starts. The land cells are based on Mean Sea Level.
You can use Grid Translator Design-a-Grid to create custom grids in an ASCII or Binary grid format for input into ArcView and ArcGIS. But you must have Spatial Analyst and/or the 3-D Analyst extension(s) installed. After creating and downloading your grid, follow these instructions:
Note: When saving your GEODAS Custom Grids for use with Arc, be sure NOT to save the grids in a folder which contains spaces. This causes errors when importing.
GEODAS Software can be used to access your data. If you did not include the software when you downloaded your data, and do not have it installed on your computer from earlier downloads or GEODAS DVDs, or if you wish to find the latest version, go to http://www.ngdc.noaa.gov/mgg/gdas/gx_announce.Html. The On-line Download version of the software can interact with data downloaded from GEODAS data sets via the web. This includes GEODAS applications Hydro-Plot (MS Windows and Linux-X86), Coastline Extractor and Grid Translator (for re-gridding downloaded grids), and ShapeFile Maker (MS Windows, for creating shape files from Marine Trackline and Hydrographic downloaded data). You may use these programs after installing the software. If the software is downloaded with the data, find the software and a readme.txt file in the setup folder.
The following grid files may be downloaded to your local disk, either by compressing them together or downloading individual files:
Please address questions or comments to dmetzger@ngdc.noaa.gov