These data were produced for the Louisiana Federal Emergency Management Agency (FEMA) Project - Phase 2 of Louisiana LIDAR Data Development: Rapides Parish, Louisiana under the Watershed Concepts contract number EMT-2002-CO-0048, Task Order 013.
dem_3009202ne.mtd - this metadata in mp (metadata parser) compatible ASCII text form dem_3009202ne.html - this metadata in HTML form dem_3009202ne_faq.html - this metadata presented as Questions and Answers dem_3009202ne.sgml - this metadata in SGML form dem_3009202ne.xml - this metadata in XML formThe information depicted on this map are the results of the survey on the dates indicated and can only be considered as indicating the general conditions existing at the time.
Concepts, Watershed, 2003, Digital Elevation Model (USGS DEM), NE quadrant of Steep Gully quadrangle, Louisiana, UTM 15 NAD83, Louisiana, UTM 15 NAD83, Louisiana Federal Emergency Management Agency (FEMA) Project - Phase 2 of Louisiana LIDAR Data Development: Rapides Parish, Louisiana under the Watershed Concepts contract number EMT-2002-CO-0048, Task Order 013. [3009202ne.dem].
This is a Raster data set. It contains the following raster data types:
Planar coordinates are encoded using coordinate pair
Abscissae (x-coordinates) are specified to the nearest 5
Ordinates (y-coordinates) are specified to the nearest 5
Planar coordinates are specified in Meters
The horizontal datum used is North American Datum of 1983.
The ellipsoid used is Geodetic Reference System 80.
The semi-major axis of the ellipsoid used is 6378137.
The flattening of the ellipsoid used is 1/298.257.
Watershed Concepts
c/o David Key, P.E
3333 Regency Parkway, Suite 120
Cary, North Carolina 27511
(919)868-5155 (voice)
DEM data are useful for terrain analysis and modeling including slope and aspect calculations. They may be used to produce shaded relief maps and contour maps.
Concepts, Wayershed, 2003, Digital Elevation Model (ArcInfo lattice), NE quadrant of Steep Gully quadrangle, Louisiana, UTM 15 NAD83, Louisiana Federal Emergency Management Agency (FEMA) Project - Phase 2 of Louisiana LIDAR Data Development: Rapides Parish, Louisiana under the Watershed Concepts contract number EMT-2002-CO-0048, Task Order 013. [lat_3009202ne].
Person who carried out this activity:
3001, Inc.
LIDAR Manager
5525 Mounes, Suite 102
New Orleans, LA 70123
(504) 733-3001 (voice)
Data sources used in this process:
The DEM has been clipped to the area of the 3.75 minute cell comprising the SE of the Steep Gully . All lattice points outside this area that comprise the balance of the minimum bounding rectangular area in UTM coordinates have the 'no data' elevation value of -32767.
The DEM was produced with an origin at an even increment of the DEM point spacing to allow adjacent tiles to be appended without resampling.
One of the primary goals of this project was to produce higher accuracy elevation data than was available for the Amite River Basin. Because of the wide array of uses for the data products, many standards had to be taken into consideration for data production. Among these were the FEMA standard for LIDAR capture and data processing, and the USGS standard for Digital Elevation Models. With regard to the USGS standard for Digital Elevation Models, the resolution of the data produced in this project is higher than that for which the standard guidelines were developed. Although this was the overriding goal of the project, it also requires the data follow the standard more loosely than if the data were spaced at the same intervals and resolution as regulated by the standard.
The USGS standard provides guidelines for 5 primary types of DEMs. The main difference between the types is the geographic coverage of the data and the resolution of GRID values within the DEM. The smallest geographic extent and grid spacing (and thus the most accurate DEM) documented in the standard is the 7.5 minute DEM (which is a typical USGS Quadrangle). The standard states that the resolution of the 7.5 minute DEM “will have a 10 or 30 meter grid spacing”. The DEMs produced in this project are tiled to USGS Quarter Quadrangles (or ¼ of the typical 7.5 minute quad). The GRID spacing is 5-meters, as opposed to the 10 or 30 meter requirement of the standard.
It should also be noted that when these DEMs are viewed with the USGS DLG Viewer software, there are inconsistencies in the elevation data reported by the viewer. These inconsistencies have not been observed in other viewers, including ArcInfo, ArcView, ERDAS and Terramodel. The reason for the inconsistencies is that the DEM generated by ArcInfo utilizes data elements of the USGS standard that DLG Viewer does not take into account when computing elevations.
Based on the USGS "Standards for Digital Elevation Models", a DEM file is organized into a series of three records, A, B, and C. The A record contains information defining the general characteristics of the DEM, including its name, boundaries, units of measurement, minimum and maximum elevations, number of B records, and projection parameters. Each B record consists of an elevation profile with associated header information, and the C record contains accuracy data. Each file contains a single A and C record, while there is a separate B record for each elevation profile.
Two data elements in particular appear to cause the rendering inconsistencies with the DLG Viewer software. The data elements are the z resolution (data element 15, record b) and the local datum for the profiles (data element 4, record b). The DLG viewer appears to ignore one of the math expressions or uses a default value to compute the elevations. The other viewers listed above can do the math and show the z-values properly. In these viewers, elevations are computed by multiplying the z-values by the z resolution and then adding the local datum value. By either using a default value for the z resolution or datum, or not taking one of these elements into consideration when computing the elevation, the DGL Viewer is not displaying the correct elevation.
According to the USGS "Standards for Digital Elevation Models" the z resolution does not have to be a multiple of 10 (e.g 1, .1, .01, etc.). This is the case for the DEMs created by ArcInfo for this project (e.g. .286508). As far as the local datum for the profiles, ArcInfo uses the lowest elevation in the file. According to the USGS Standard, the local datum does not have to be set to zero.
Are there legal restrictions on access or use of the data?
- Access_Constraints: none
- Use_Constraints:
- These unaltered data may not be redistributed without all of the elements of the metadata listed in the Supplemental_Information section of this metadata document.
3001, Inc.
5525 Mounes Street, Suite 102
New Orleans, LA 70123
(504) 733-3001 (voice)
(504) 734-8938 (FAX)