Digital Scales Articles

FEMA Goes Digital

Wendy Lathrop, LS

http://www.profsurv.com/ps_scripts/article.idc?id=115

As everything goes digital these days, technology is unveiling new means for conquering formerly formidable challenges. Surveyors use digitizers, collect data in digital format and manipulate digital files. Some of the new products available are major improvements over the old ways of doing things, but unwary first-time users going digital should realize that data collected manually and put on a computer often looks better than it is. Think back to that first new calculator with eight places to the right of the decimal instead of only four. Was it really any more accurate? The Federal Emergency Management Agency (FEMA) has jumped on the digital bandwagon as well, with mixed results. Surveyors should welcome the trend, but at the same time they should keep a wary eye on digitized FEMA mapping products until some bugs have been worked out.

The base mapping for Flood Insurance Rate Maps has always been an issue of contention. The specifications for study contractors merely stipulate that the “best available” base map is to be used, but there are no standard criteria for deciding what is good enough and what should be rejected, even if it is the best available. Consequently, in the early 1970s some of the original flood mapping was overlaid on traced USGS quads (1:24.000) or, even worse, street maps that were blown up to designated scales. Unfortunately, I know this because I worked for a study contractor at that time, and it grieves me to see that what were supposed to be maps for just the emergency phase of entry into the National Flood Insurance Program are still the most current documents more than 20 years later.

The Digital Flood Insurance Rate Map (DFIRM) is a digitized version of the paper copy of the FIRM. Specifications are consistent with mapping at a scale of 1:24,000. This is useful for those with a GIS based on the USGS 7.5-minute quadrangle. However, quality for enlargements to the scales that are generally utilized for engineering and surveying projects will decrease proportionately.

TIGER Files Used

Because of complaints about lack of street detail on flood maps (meaning, no streets shown outside of the flood areas) or street labels, FEMA investigated Topologically Integrated Geographic Encoding and Referencing (TIGER) data, and decided that this digital base mapping was a great saver of time and money. Many municipalities made this same assumption when starting their GIS base, and then ended up spending a lot of time and money correcting, or as they say euphemistically, “enhancing” the TIGER files to get a semi-realistic picture. TIGER was created during the 1990 Census, being a compilation of street segments with attributes such as address ranges and street names. It was never intended to be used for engineering applications such as flood studies and mapping, but was a useful collection of data to help analyze demographics.

Flood Risk Directories

Another outcome of the digital attempts with TIGER has been the creation of Flood Risk Directories (FRID). These are phone-book-style listings of addresses with notations as to whether properties are in or out of the Special Flood Hazard Area (100 year flood zone). The information is based on a TIGER file overlay onto the Flood Insurance Rate Maps. Unfortunately, TIGER data is arranged in address ranges and proportioned throughout a block so that every lot is viewed as having the same frontage. This is not particularly accurate; lots do not have the same frontage, and structures on “flag” lots may be set back considerably further from the roadway than buildings on adjacent lots.
For several years, FEMA has been experimenting with different data sets to create new base maps. For some of the new county-wide mapping, which was intended to overcome the errors associated with mismatches of flood data and streets at community boundaries, FEMA has relied upon USGS Digital Line Graphs. However, DLGs are at 1:100,000, and the flood map scale may be 1"=500'. The distortion of enlarging 1,667 percent does not need to be elaborated upon, as I am sure most surveyors can easily understand what even a line width displacement will do with that kind of extrapolation.

To address such difficulties, FEMA has put a note on the new digitally based flood maps: “Map users should be aware that this base map source causes road alignment distortion at and near road intersections. These alignment problems have been corrected in the vicinity of identified floodplains.” Users beware. The process results in jagged street segments, peculiar configurations, odd intersections, and water-course shifts. One of the Technical Evaluating Contractors assured me that they have learned from the first such map but I remain unconvinced of the merit of enlarging even from 1:24,000 to 1"=500'.

User Can Create Basemaps

FEMA’s Q3 (quality level 3) product is a digital form of the flood layer of Flood Insurance Rate Maps. Users must provide their own base map (and their own mapping software) onto which the flood layer is to be registered. This product was designed for in-house use at FEMA for disaster response and risk assessment. The Q3s are not edge-matched, nor are overlaps or gaps in coverage corrected. Horizontal accuracy is intended to be consistent with National Map Accuracy Standards for mapping at a scale of 1:24,000 (1"=2,000') and is assumed to be no better than 40 feet. Recommendations have been made that there is a 250 foot margin of error or “buffer zone” associated with the location of the Special Flood Hazard Area limits. Fact sheets on Q3 flood data published by FEMA and documentation for the Q3 demonstration CD-ROM (issued in May 1996) emphasize that “Q3 Flood Data can not be used to determine absolute delineations of flood risk boundaries, but instead should be seen as portraying zones of uncertainty and possible risks associated with flood inundation.”

Features in the Q3 flood data files are the 100 and 500 year flood plains, zone designations, floodway boundaries (when available), political boundaries, community and map panel identification numbers, panel neatlines, and USGS 7.5 minute (1:24,000) quad neatlines. Data not contained in the files are hydrographic features, base flood elevations, cross section lines, roads or road names, and bench mark locations or elevations. More information is available on the Q3 on FEMA’s Internet site (http://www.fema.gov) and from the Map Service Center in Baltimore (800/358-9616).

Digtal Maps Have Benefits

Despite all these difficulties, the reason for going digital is obvious—time and cost savings. In fact, when a good base map is finally attained, the product will improve appreciably and the updating process will be faster and simpler. Many maps now in circulation are scribe-coat based, meaning that instead of using a photographic negative, lines were scratched into an opaque surface. This technology is obsolete, and scribe coating materials are now nearly impossible to find.

A digital product allows for correction of a small area without having to shoot a whole new negative. It also allows for “print on demand” so that maps can be sent out immediately (no more back orders!) and no inventory needs to be maintained. Imagine the cost savings in accomplishing the update from Letters of Map Correction or new flood studies, in reduced storage space, in speedy retrieval and distribution of the new map. FEMA has the right idea. We can only hope that the Agency listens to recommendations of the Technical Mapping Advisory Council (established by the National Flood Insurance Reform Act in September 1994), holding product quality and long-term savings in higher regard than short-term apparent cost reduction.

Wendy Lathrop is the Manager of the Geographic Search Services Division of Charles Jones, Inc. in Trenton, New Jersey and a Contributing Editor for the magazine. She is also the current President of NSPS and the ACSM representative to the Technical Mapping Advisory Council to FEMA.