. The effect of structures and lake level on bluff and shore erosion in Berrien County, Michigan, 1970-74 . 0 " ^ K " Sample Size {meosuremenf points per Km) Figure A-2. Comparison of maximum difference between Xg, the mean bluff recession rate of a sample of n equally spaced measurements per kilometers, and x^y, the mean computed for all 57 stations. Lines are shown for 1- and 4-year periods (curves are hand-fit). From Figure A-2, it appears that for data from 1-year periods, 20 measure- ment stations at equally spaced increments per kilometers would result in a mean rec
. The effect of structures and lake level on bluff and shore erosion in Berrien County, Michigan, 1970-74 . 0 " ^ K " Sample Size {meosuremenf points per Km) Figure A-2. Comparison of maximum difference between Xg, the mean bluff recession rate of a sample of n equally spaced measurements per kilometers, and x^y, the mean computed for all 57 stations. Lines are shown for 1- and 4-year periods (curves are hand-fit). From Figure A-2, it appears that for data from 1-year periods, 20 measure- ment stations at equally spaced increments per kilometers would result in a mean recession rate within ± meter per year of the value which would be determined using 57 stations. This amounts to a 65-percent reduction in the number of measurements needed. Since Figure A-2 is based on the maximum difference between (instead of the mean difference), it should be conservative. and ^5 7 Fewer stations are needed for longer period data as indicated by the shift of the line for the 4-year data to only 10 stations required for the same accu- racy. If an accuracy of ± meter per year is inadequate. Figure A-2 can also be used to select a larger san^le size. This rather simplified procedure for estimating measurement station fre- quency is presented for general guidance in setting up a similar study. The actual number of stations needed will vary depending on the quality and scale of the air photos, the time period considered, the accuracy desired, the uni- formity of bluff type, and the analysis procedure. It is suggested that some experimenting be done with measurement density along a short reach of coast before a final station frequency is selected. 74
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