n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

~ t I r n t S i m ~ ~ Alarr a ~ many owho f helped to develop the teehnlques used In the
lrsn gZavtty data
oontermhous 48 States. Particularly significant ware
the contributiom of: (1) O. P. W ooUard h Thiel and
others, 3958), who recognized that Alaskan maps requlred
the identfflcation of elevation sources in com-
The recent avallabiIity of an almost complete puter files1 (2) Donald Plouff (1968, 19771, who
file of digital elevation rncdels (DEM1s) for the State developed a fundamental computer terrain-orrection
of Alaska makes possible new maps and calculations, program, using a geographic grid; (3) Atef Elasgal
including the terraln and &static correction of the (written commun., 19751, who prodded a mend of
gravity data wed to are the previous gravity map reading plmar-elevation-model tapes and calculating
of Alaska (Barnes, 1877 "P . This refinement of data, mean-eompartrnent elevations; (4) Donald Plouff, who
once sultable only for smaU%ale regional maps of adapted 3lassal's programming to his terT.ralnmoun~ous
areas, now makes the same data useful correction system; and (5) R. L. Godson (written
for larger scale maps and for such quantitative lnter- commun., 1980), who integrated many of the conterpretative
procedures as digital mdeling. Tl~e DWts minou&l&States contrlbutfons into a user-oriented
consist of metric elevations on a nearly square (com- system of gravity reduction designed for the USGS's
monly 3 by 6 second) geographic grid derived from Honeywell Multics computer. The present USGS Alas-
1 :250,000-saale topographic maps and available on kan system, following a parallel development assisted
magnetic tapes distributed by the U.S. Geological by these and other contributors, now handles the pro&
Survey's (usGS) National Cartographic Information lems peculiar to Alaskan gravity and elevation data,
Center in Reston, Va. (documentation available from but It WI far operates Drily on the USGS MultIcs system
that offlce). New computer techniques to read these (a largescale virtual-memory system designed primartapes
now permit terrain corrections On the gravity ily for interactiva low-speed use, although programs
data accumulated over the past 20 to 30 years, and written for it may be difftoult to bansfer to other
future programs wiU provide isostatic, mean-elevation, systems).
and other maps. Use of digital models for terrain cor- The problems that caused the computer precessrections
on gravity data from the conterminous 48 ing of Maskan gravity data to differ from procedures
States has been possible for the past 0 years, but the used in the conterminous 48 States became evident
procedures used on Alaska data are newer and not yet during the earllest USGS Alaskan gravity surveys,
so well perfected.
which began in the Copper River basin in 1958. Those
The differences in Alaskan procedures wult first-year measurements revealed a broad but poorly
from several factors, including: (1) the soarcity of defined area In which altimeter elevations differed
Aleskan survey elevation control, whioh has prevented from spot elevations on 1263,360-scale maps by as
moat topographic maps from meeting national map- much as 15 m. Repeated altimeter measurements the
accuracy standards; (2) the resulting abundant use of following year and a leveling Une several years later
altimetry; (3) the convergence of meridians at high confirmed the accuracy of the altimetry, although the
latitudes; and (4) the DEM format of the elevation cause of the spot-elevation errors has never been
tapes, which are now available for Alaska. In contrast, determined. Similar spot-elevation errors were later
data-reduction and terrain-i?orrectim pmedures f6r found in other partrc of Alaska, although poor weather
the conterminous 48 States have been influenced by conditions on some days also caused altimeter meassuch
factors asr (1) the conformance of most tow urements to be almost equally erroneous. The most
graphic maps to national map standards, (2) the neerly consistent contours were obtained from altimeter
square gecgraphlc grid at low latitudes, (3) a file of measurements, but only map elevations could be used
manually averaged mean-compartment elevations for accurate terrafn corrections. Thae problems sug-
(mostly 1,000-yd or I-km grids plus 1- and 3-minute gested that the best remits would be obtained by
grids) that gradually accumulated between 1960 and maintalnlng both dtimetry and mapderived elevations
1980, and (4) the preliminary planar format (elevations in the computer data file aod labeling the sources of
on a Cartesian grid requiring map-projection transfor- all the elevations. The tendency of altimeter elevemations)
of the early digital elevation tapes.
tiona to worsen on individual days of poor weather also
The Alaskan techniques have profited greatly suggested that each day's data files should be sepafrom
papers, discusslone, and communlcatlons from rately accessed and that all computer listings should
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J?igue 2.-ALaska, showing quadrangles for which
Landsat studies have been completed. NORTHERN
ALASKA: A, Philip Smith Mountains quadrangle (Le
Compte, 1079a); 0, Survey Pass quadrangle (Le
Compte, 1981f); C, Ambler River quadrangle (Albert,
1978). WESTSENTRAL ALASKA: D, Medfra
quadrangle (Le Compte, 1981e). SOUTHWBSTBRN
ALASKA: E, Lake Clark quadrangle (Steele, 1983);. F,
Goodnews and Hagemeister Island quadrangles reglon
(Steele, 1978); G, Ugashik and Karluk quadrangles (Le
Compte, 19811); H, Chignik and Sutwik Island
quadrangles (Le Compte and Steele, 1981). EAST-
CENTRAL ALASKA: I, Chandalar quadrengle (Albert
and others, 2978); J, Ciwle quadrangle (Le Compte,
1981a); K, Big Delta quadrangle (Albert and Steele,
1978); L, Tanacross quadrangle (Albert and Steele,
1976b). SOUTHERN ALASKA: M, He* quadrangle
(k Compte, 1981h); N, Talkeetna quadrangle (Steele
and Albert, 1978x1); 0, Talkeetna Mountains quedrangle
(Steele and Le Compte, 1978); P, Nabesna quadrangle
(Albert, 1975); Q, McCarthy quadrangle (Albert and
SteeIe, 1976a)i R, Valdez quackmgle (Le Compte,
1981g)~ S, Seward and Blying Sound quadrangles (Le
Compte, 1979b). SOUTHEASTERN ALASKA: T,
Petersburg quadrangle and vicinity (Le Compte,
1981~); U, &adfield Canal quadrangle (Le Compte,
1981d); V, Ketchikan and Prince Rupert quadrangles
ateele and Albert, 1978b; LeCompte, 1981b).