Aquifer data: Explanation of spatial data formats

These data are from regional hydrologic models that were developed to assess the availability of the Nation's groundwater resources. The models are constructed such that the hydrologic system is discretized with a grid of cells which are identified in terms of rows, columns, and layers. Many of the data sets provided here are organized into grid cells that correspond with the rows and columns that were defined by each hydrologic model. This approach allows for direct access to data that were used for model calculations and allows the scientific and water management community to more easily update these models with new data, apply the models in different ways to answer new water-management questions, and create new local-scale models.

Several formats of each data set are provided in order to meet the needs of multiple users. These formats are described below.

Shapefile: A georeferenced file format for storing the location and attribute information of geographic features. Geographic features in a shapefile can be represented by points, lines, or polygons (areas). This file format is compatible with ESRI® ArcGIS and can be converted for use by number of other software packages. In addition, any software that can read Dbase files will be able to access the attributes of a shapefile. Hydrologic model data can be visualized in "real world" coordinates with this format.

Text File: A non-proprietary file format. Tabled data values are delimited by commas or tabs.

Raster: A data format consisting of an array of equally sized cells arranged in rows and columns of a matrix (or a grid) where each cell contains a value representing information, such as elevation.

ASCII Grid (Model Resolution): A text file interchange format that contains a header specifying the geographic origin and resolution (cell size), and a value (for example, elevation) for each grid cell. This file format is the same resolution as the model and is convenient for archiving hydrologic model data in a non-proprietary format, but not necessarily for display in "real world" coordinates.

ASCII Grid (High Resolution): A text file interchange format that contains a header specifying the geographic origin and resolution (cell size), and a value (for example, elevation) for each grid cell. This file format is convenient for archiving hydrologic model data in a non-proprietary format. It uses a higher resolution (typically 10% of the model cell size) to more accurately represent data that have been generated from a model grid that is not in a North-South, East-West orientation. This format should be used for spatial analyses where data must be represented accurately in "real world" coordinates.

Geodatabase: A collection of data layers of various types that are stored in a single database. This format is the native data structure for ESRI® ArcGIS. Hydrologic model data can be visualized in "real world" coordinates with this format. A geodatabase provides a structure for data to be efficiently stored, organized, queried, and managed.

Metadata: Metadata describes information about a dataset, such that a dataset can be understood, re-used, and integrated with other datasets. Information described in a metadata record includes where the data were collected, who is responsible for the dataset, why the dataset was created, and how the data are organized. Metadata follows the Federal Geographic Data Committee (FGDC) standard format, making it easier to compare datasets and to transfer files electronically.