Beside the map books, each assessor should have field books containing columns for the final assessed values for several years; a wide column for remarks, and columns showing the name of the owner, if known, the size of the lot, the number of houses on the lot, the size of each house, the number of stories in height, the street number and the lot number. Above each block should appear section and block numbers, and the number of the volume to correspond with the number of the volume of the assessment roll. In New York each section is divided into volumes and the volumes are numbered from one up consecutively for each section. The volume numbers change from time to time as it may be necessary to increase the number of books in which the record is made.

Part of the office system should be the preservation of all records giving evidence of value. The card index system is probably the most convenient, and this should show, as nearly as may be, all conveyances with the consideration, and if the number is inadequate, a record should be kept of all mortgages, contracts of sale and more important leases. In the remarks column of his field book the assessor should set down such matters of record and any further information in regard to value that he himself can secure. On both the assessment roll and record book there should be a separate column for the value of land, exclusive of improvements.

When the assessor is equipped with map and field book filled with all available data, he should first determine the value of the land per front foot for the unit of depth, which in New York is 100 feet. It will generally be found most convenient to set down this unit value of land on the key map in the map book. Having determined the front foot value of land 100 feet deep, the actual value of each lot is very quickly ascertained. When lots are irregular in width and depth, when lots are shorter or deeper than 100 feet, the value must be ascertained in accordance with a scale which experience shows is suitable for the particular city. It is always the case that land near a street is worth more than land further from it, for all city land is valued in proportion to the character of the street on which it fronts. A city block may be defined as a parcel of land entirely surrounded by streets, and the nature of this street frontage determines the value.