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DoC/FEMA Central California Mapping
Note on STAR3-i Data
This data is supplied in it’s unenhanced form to allow for the wide range of applications this data may be used for. Usage and enhancements are specific from application to application; and, as such, performing one enhancement may preclude accurate interpretation of data for other applications. As a consequence of this, InterMap does not enhance the final product thereby allowing the client to define the enhancement best suited to the application and also to facilitate further use of the data for other different applications.
As with most radar magnitude data, a low dynamic range in the return signal results in an extremely dark image. This is normal to radar magnitude data and can be adjusted using simple image analysis enhancements, such as Linear and Root Stretches.
File Naming Convention
Each file has been given a unique eight (8) digit file name using the DMA World Reference
System (WRS) and as used by the USGS. This file naming convention corresponds to the geographic location of the South East corner of the map tile. The file name prefix denotes the file type. Each map tile is a 7.5’ x 7.5’ tile, there are 64 tiles that make up one, 1 degree x 1 degree cell. The USGS publication, California Index to Topographic and Other Map Coverage, publication 38121-E4-MI-99X, is an excellent guide for locating maps using this reference system.
For example, a map tile covering Stockton California would be 137121H2.
The file name is read as follows:

First Digit 1-4, the globe has been divided into four quadrants,
1=NW, 2=SW, 3=SE, 4=NE

Second and Third digit: Latitude 1 degree intervals valid range 0-90
Fourth to Sixth digit: Longitude 1 degree intervals, valide range 0-180

Seventh digit: alpha character A-H, Row Numbers from south to North

Eight Digit: Column Number, 1-8, East to West
More Particulars to this Data Set
Each DEM or Magnitude file has a short .txt (text file) for its particulars for each file. This .txt file is named using the WRS number for that tile, and whether it is DEM or MAG data.
Each CD has an extensive notation file with more complete information for the contents of each
CD and its files -- with more particulars one needs to geo-reference, etc. each file. Each of these
CD .txt files is named using the eight digit InterMap number for that CD. There is a list of the files referenced by InterMap number, for which reference data is noted in that text file.
WARNING or a word of caution.
DO NOT INTERCHANGE .txt files for DEM or MAG files. There is a short .txt file for each DEM and Magnitude image, they are not the same!
Corner coordinates, can to be different and unique to each DEM and magnitude even though they are a pair... Do not assume rows and columns remain constant, they do change occasionally.
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Product Description
Data Source:
Sensor:
Band:
Format:
Motorola Processor
Intermap Processing Level:
Image Pixels:
DEM posting:
Horizontal Accuracy:
Vertical Accuracy:
Flying Height:
Primary Look:
1 st Acquisition Date:
2 nd Acquisition Date:
Geographic Parameters
Projection:
Horizontal Datum:
Vertical Datum:
Geode Model
Central Scale:
UTM Zone:
Central Meridian:
False Easting (meters):
False Northing (meters):
Example DEM Header
DEM File
Projection
Easting (meters):
Northing(meters):
Pixels (columns):
Lines (rows):
Pixel size (meters):
Pixel origin:
Example IMAGE Header
Image (Magnitude) File:
Projection: upper left Easting: upper left Northing: number of rows: number of pixels: pixel size:
Airborne Interferometric Synthetic Aperture Radar (IFSAR)
Intermap Star3-i (previously ERIM IFSAR-E)
X-band
32 bit BIL (band interleaved) swap bit for Intel based processors
GT2 (vertical RMS 2 meters)
2.5 meters
10.0 meters
2.5 meters (1 sigma)
1.5 meters (1 sigma)
20,000 ft. Above Sea Level
North East (mission 66)
Northern portion August 1997
Southern portion May 1998
Universal Transverse Mercator (UTM)
WGS84
Mean Sea Level
EGM96(NIMA96)
0.9996
10 (two tiles are recast into zone 11)
123 degrees West
500,000.0 meters
0.0 meters
137120A5.DEM
UTM Zone 10
Min. 710605.0 Max. 722825.0
Min. 4097445.0 Max. 4111745.0
1,223
1,431
10.0 center of pixel
138122G1.MAG
UTM Zone 10
575281.0
4303541.0
5720
4888
2.5 meters
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Area of Coverage
FEMA Flood Project
Radar Terrain Data
Figure 1. Location Map of FEMA Flood Data
Coverage of the FEMA Central Valley Flood Project data set extends from north of Sacramento southerly down to Fresno, following the Sacramento and San Joaquin Rivers. The project area covers about 17,000 square kilometers (or 6,564 square miles).
Project Background
During the winter floods of 1996-97, a proposal was developed by the California Department of
Conservation (CalDoC) to use the STAR-3i airborne X-band radar terrain mapping system operated by Intermap, to acquire IFSAR DEM data. The proposal was funded by the Federal
Emergency Management Agency (FEMA), with the Army Topographic Engineering Center (TEC) as the project manager and with CalDoC providing user input.
Based on experience modeling terrain with IFSAR generated DEMs for the GeoSAR Project, and from having worked with ERIM IFSAR-E data, CalDoC proposed a flight plan to conduct an
IFSAR data collection of the Sacramento and San Joaquin Valleys. The goal was to develop a
DEM of the major river channels and their levees in the Central Valley, along with the adjacent flood prone areas. This could be used for advance planning and improved response in real time.
The intent was that by using an IFSAR DEM along with GIS, one would be able to model the extent of flooding into adjacent areas if a levee were to be breached.
The STAR-3i System
The Interferometric Synthetic Aperture Radar for Elevations (IFSAR-E), was developed by the
Environmental Research Institute of Michigan (ERIM) and resulted in the fabrication of an interferometric radar on a Learjet 36A. NASA’s Jet Propulsion Laboratory (JPL) developed processing software and the ground-processing environment. This software and ground processing capability has now been transitioned to Intermap Technologies, the system is now referred to as the STAR-3i. (see http://www.intermap.ca/HTML/mapp_star3i.htm)
The STAR-3i system consists of two X-band radar antennas. Data collection from the twin antennas occurs simultaneously. The data are “interfered” by a digital correlation process to extract terrain heights that are used to geometrically correct the radar image. STAR-3i uses postprocessed differential GPS, together with on-board inertial navigation to obtain highly accurate positioning control. Terrain height and positioning data are enhanced by calibration of the baseline (the distance between the two antennas). The accuracy of the positioning information and calibration is such that no in-scene control points are required. The only requirement is that
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a ground-based GPS receiver must be located within 200 kilometers of the data collection site so that differential GPS processing can take place.
Three kinds of data, or files, are generated:

Elevation File - the elevation data, or DEM, provides information on terrain elevation and height of features.

Magnitude File - a backscatter image that provides information on the shape of features, as well as terrain texture.

Correlation File - the correlation image provides information on surface or volume backscatter. Note: there is no correlation data provided for this particular data set.
The STAR-3i is typically flown at 12,000 meters and acquires a 10-kilometer wide swath of 2.5meters resolution on the ground. The system has been designed to collect DEMs at a rate of 100 square kilometers per minute with 3-meter accuracy.
Improved DEM accuracy is achieved by reducing the aircraft altitude to 6,000 meters, which reduces the swath width to 6 kilometers. At this lower aircraft height, ground resolution stays the
 same; however, the signal-to-noise ratio is one-half that of the higher altitude, thereby improving precision in the vertical direction.
Advice To Users

The following comments and suggestions are provided so that future users of IFSAR data can better understand some of the operating characteristics associated with this technology. .
 Radar DEMs do not portray a bald earth. Vegetation and large cultural features are included in the DEM. In some cases this may be desirable, such as for locating trees or riparian vegetation along a stream. In areas such as dense forest or where buildings are mapped, the ground surface will be obscured and the DEM height will be that of the features. Use of a
DEM with these “first reflected surfaces” may be of great utility for construction of orthorectified digital imagery. Various approaches are being investigated and developed to generate a true ground surface or bald earth DEM.
Flight path orientation is an important factor in which two problems with the return signal can occur:
 no return signal or “dropouts” from terrain shadowing, radar has a tendency for shadowing just as with optical photography; and,
 an overly strong reflected signal or “blossoming” from manmade structures.
The flight path should be aligned to minimize terrain shadowing--which is effected by a combination of sensor altitude, slant range angle and terrain height. In addition, the flight path should be at a slight angle to the urban pattern, to minimize what can be described as a blossoming of the return signal from structures facing the radar antenna. If buildings or structures are perpendicular to the flight path, this will produce a reflected signal that is too strong for the system to process, resulting in buildings being “whited out”. Both these problems can be minimized or eliminated with forethought and careful flight line planning.
Flood modeling requires a common vertical datum (such as sea level); it is suggested that orthometric heights be used. The user’s GIS model and the radar DEM must both have their elevations referenced to the same datum if they are to be combined. Otherwise there is a danger of a water level or elevation in one terrain model not matching the same elevation in another model.
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Future Efforts
It is hoped that by providing this data to users they will be exposed to the utility of radar DEMs and provide feedback to improve future data collection and applications.
A set of requirements needs to be developed to define features, dimensions, and the precision and accuracy to be obtained for flood modeling applications; and, other applications that a radar
DEM or image may be suitable for. A next step will be collecting information from users toward this effort.
For comments, suggestions and/or questions, please contact:
Robert Yoha
Office of Technology Assessment, Planning and Development
California Department of Conservation email ryoha@consrv.ca.gov phone (916) 322-9307
Carlton Daniel
U.S. Army Topographic Engineering Center
Terrain Data Representation Branch email daniel@tec.army.mil phone (703) 428-6838
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