At the conclusion of the hearing, the board shall by resolution determine whether it is necessary to form the improvement district. If so, the resolution shall also state:

1. (a) The purpose for which the proposed improvement district is to be formed.
2. (b) The estimated expense of carrying out such purpose.
3. (c) That the exterior boundaries of the portion of the district which will be benefited are set forth on a map on file with the secretary, which map shall govern for all details as to the extent of the improvement district.
4. (d) That such portion of the district set forth on the map, shall thereupon constitute and be known as “Improvement District (A, B, C, or other letter designation)____ of ____ Municipal Water District.”