Laserfiche WebLink
MEMO 93 <br />7. Comments and Concerns <br />The accuracy of irrigated parcel delineations, associated crop types, and water sources reduced slightly <br />from 2001 to 1956 as a function of both lower quality available imagery for each time period and overall <br />lack of ground data and water user inputs. Specifically, limitations of available historic imagery <br />(resolution, multi-spectral imagery vs. panchromatic photos, and bit depth) increased uncertainty in <br />identifying changes in crop patterns and agronomic practices. Compared to 2001 imagery, the lesser <br />quality imagery is most apparent in the 1956 aerial photo indexes. As discussed in SPDSS Technical <br />Memorandum 93.1, the 1956 aerial photo indexes are a collage of manually arranged aerial photo prints <br />for each area of aerial photo coverage and is, therefore, sub-standard compared to modern imagery <br />systems. <br />The limitations in 1956 aerial photo indexes available for irrigated parcel determination caused more <br />uncertainty in the 1956 data set (parcels, crop types, water sources) than the 1976 or 1987 data sets when <br />multi-spectral, multi-date imagery was available (as discussed in SPDSS Technical Memorandum 93.1, <br />Section 3). Procedures used for 2001, 1987, and 1976 irrigated parcel delineation and crop type <br />classification using multi-date, multi-spectral imagery was not possible from the single date 1950s black <br />and white aerial photo indexes.. However, irrigated lands were confidently identified for the 1950s <br />through manual photo-interpretation using the black and white aerial photo indexes and information <br />regarding historic diversions, ditches, and wells. The crop types for the 1950s irrigated parcels were <br />copied from 1976 irrigated parcels (as discussed in Section 4.2.1.3). This gave a reasonable estimate of <br />the 1956 crop types. <br />Surface water source was assigned to 1987 historic irrigated parcels using 2001 irrigation service areas as <br />a starting point. Final 1987 surface water sources were the starting point for 1976 water sources, and <br />final 1976 surface water sources were the starting point for 1956 water sources. Many diversions that <br />were inactive and unmapped in 2001 were mapped in the historic years. Although water users or ditch <br />companies did not verify the service areas for the previously unmapped diversions, they were delineated <br />using the same procedures reported in SPDSS Technical Memorandum 91, Section 2.2. RTi believes the <br />quality of the previously unmapped service areas to be good. <br />The task of mapping irrigated lands with remotely sensed data for historic time periods presented unique <br />challenges not encountered when mapping current irrigated lands. The scan line errors and coarse spatial <br />resolution of the 1976 Landsat MSS data required additional processing steps and supplemental data for <br />use in mapping activities. Scan line correction algorithms were applied to affected MSS data that helped <br />minimize scan line impacts on classification results. The coarse resolution of the 1976 Landsat MSS data <br />was overcome by using the DOQ aerial imagery as a guide to update the 1976 irrigated parcel boundaries. <br />The 2001 crop type classification procedure was adapted for the historic crop type classifications to <br />overcome the absence of crop type reference data. Knowledge gained about crop phenology and spectral <br />reflectance during the 2001 growing season was applied to the historic time periods. This experience <br />enabled classification of crop types without parcel specific crop type reference data. The lack of parcel <br />specific crop type reference data also prohibited formal accuracy assessment of classification results. <br />Concerns and solutions were discussed with the State throughout the duration of the project. The <br />implemented solutions effectively overcame many of the limitations and concerns with the data. The <br />resulting irrigated parcel and irrigation service area datasets for 1956, 1976, and 1987 are high quality <br />estimates of the irrigated lands for each time period in the South Platte and North Platte River Basins. <br />Page 25 of 59 ,Rrvera~Ge Feel~n81'agy, ioe. <br />