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MEMO 93 <br />Not all irrigated parcels were updated using printed maps. Areas within the SPDSS containing <br />irrigated parcels outside the printed map areas were updated onscreen. After all of the printed <br />maps had been processed, and all areas outside printed maps had been updated, final QA/QC <br />procedures checked all aspects of parcel updates for all historic mapping periods. <br />4.1.2.2 1950s Parcels <br />Aerial photo indexes were used for 1950s irrigated parcels updates and, for the North Park area (Division <br />6, District 47) both the indexes and individual aerial photos were used. 1950s irrigated parcels were <br />updated manually on screen by means of heads-up digitizing from the 1950s aerial photo indexes along <br />with consideration of ancillary data sets (ditches, wells, service areas, digital elevation models). <br />The first step in updating the 1950s parcel layer was the addition of "new" 1950s parcels (parcels not <br />present in the 1976 parcels data layer). New parcels were identified most commonly in and around urban <br />areas. Areas of greatest change in agricultural activity were along the Front Range and specifically near <br />the cities of Greeley, Fort Collins, Loveland, Longmont, and the greater Denver area. The addition of <br />new 1950s parcels required photo interpretation of irrigated status. Irrigated versus non-irrigated status <br />was manually interpreted from aerial photo indexes, ditch service areas, topography, and water <br />availability (proximity and access to ditches and wells). New 1950s parcels were added allowing for the <br />offset in both scale and location of the aerial photo index. Figure 6 shows an example of 1950s parcels <br />relative to a geocorrected aerial photo index (a) before and (b) after further analyst processing. As <br />described in Section 2.1 of this memo, the analyst further processed 1950s aerial photo indexes after geo- <br />referencing by adding a single control point using the ArcMap georeferencing toolbar. The single control <br />point automatic adjustment in ArcMap significantly improved the placement of the aerial photo index. <br />The analyst would then digitize the outline of the new parcel, often digitizing large blocks of irrigation <br />containing several parcels. Newly added 1950s parcels were not segregated to the same detail as 2001, <br />1987, or 1976 irrigated parcels due to relatively low quality of aerial photo indexes. Therefore, newly <br />added 1950s parcels were delineated by interpretation of distinct field boundaries such as roads, ditches, <br />and fence lines. <br />(a) <br />(b) <br />Figure 6. Example of 1950s parcels relative to a section of geo-referenced aerial photo index (a) before <br />adding a single control point and (b) after adding a single control point. <br />Secondly, center pivot irrigation technology was not available during the 1950s and, therefore, all center <br />pivots were modified to reflect 1950s agricultural field boundaries, or removed if no obvious agricultural <br />activities were present. <br />Page 11 of 59 ,Rereradde ~eshn~P~gy, ~os~ <br />