Base Map Preparation Process -2

Natural and cultural features that are relatable to a cadastral parcel form the next most important levels of base map data. One of these levels includes all streets, roads, railroads and airports, with their associated names. Another level includes all permanent buildings and other structures greater than a specified size. A third level includes all water features such as perennial and intermittent streams, natural and man-made lakes and ponds, reservoirs, canals and aqueducts and their associated names. A fourth level includes boundaries of civil (Governmental) jurisdictions at all levels: state, county, city and township. Other secondary levels of natural and cultural features, such as contours. floodplains, wetlands, vegetation cover, land use, and utility lines, may be included selectively in the base-map composite. 

Use of a number of different levels or overlays of base map data is essential to provide flexibility in meeting the different requirements of different map users. Drafted overlays must be precisely registered in position to each other as illustrated in Fig. A planner may desire, for example, to use as a working base a composite of the land use, floodplain and base map overlays. The greatest flexibility in base map content, to satisfy user requirements, is in digital mapping. Map information can be separated digitally into a maximum number of data levels, updated most efficiently, and plotted precisely on a single base sheet using any specified number of data levels as required. At the same time, standards and procedures must yet be established to control the level of map content detail as map scale is changed over wide ranges. 

Overall, the base map that supports a multipurpose cadastre must provide as a minimum enough planimetric detail for locating ownership boundaries referenced to natural features, such as stream and lake shorelines or to man-made features not as yet tied to the coordinate system, such as highways and railroads. Desirably, it should show all objects related to the location of real property boundaries, such as fences or driveways, at reasonably frequent intervals.

Fig  A registered overlay system

Accuracy of the horizontal and vertical position information on the base map is fundamentally a function of the map scale and contour interval, respectively. National Map Accuracy Standards have long been used as the primary standard to control the accuracy of plotted map information. For scales larger than 1:20,000, which include essentially all base maps that would be used to support a cadastral overlay, standards for horizontal accuracy specify that 90% of the points tested shall be plotted on the map within 1/30 inch of their true position. Standards for vertical accuracy specify that 90 % of the points tested shall be shown in elevation within one half of the contour interval used on the map.

The Photogrammetry for Highways Committee of the American Society of Photogrammetry has prepared specifications for large-scale mapping for highways, with a horizontal accuracy requirement that 90% of all planimetric features be plotted within 1/40 inch of their true position. This is a more stringent requirement than the comparable 1/30 inch required by National Map Accuracy Standards and has also been suggested by the Task Committee for Photogrammetric Standards of the American Society of Photogrammetry in their recently proposed accuracy specifications for large scale line maps. Either the 1/30 inch or 1/40 inch requirements have been adopted by nearly all users in their base mapping specifications for large scale property ownership maps. 

The requirement that the base map of a local record system be compiled according to National Map Accuracy Standards is primarily due to the need for the base map to satisfy the engineering needs of public works departments. When accurate information is necessary, specific boundary lengths would come from a recorded plat, boundary description or other report of survey, not from scaling the cadastral overlay on the base map. A new Engineering Map Accuracy Standard has been proposed by the Committee on Cartographic Surveying of the Surveying and Mapping Division of the American Society of Civil Engineers (ASCE). These standards are intended to provide a clearer communication of accuracy requirements between those having the need for the map and those preparing the map. Also included are specific field testing procedures to assess the compliance of the map with the standards. 

The scale of the cadastral map system is principally a function of the size of the predominant land parcel. This criterion generally corresponds to the level of land value or degree of urbanization. Listed in Table 1 are the scales that have been selected almost universally for each type of area. Contours may only need to be included on the base map for specific users with a requirement for topographic detail. The added expense is substantial. Contour interval would be selected in conjunction with the map scale, the terrain relief, and the elevation information requirements. Typical combinations are listed in Table 2.

Table 1 Suggested base map scale

Table 2 Appropriate contour intervals for suggested map scale

Necessities of a Base Map 

  • To give a place to record our observations and interpretations of the site
  • To give us a canvas for drawing our concept, schematic and detailed designs
  • To reduce the time we have to spend redrawing the permanent elements of the site 
  • To provide consistency between the different plans & drawings we are going to present to other people It gives us a not quite “blank canvas” that we can quickly duplicate
  • Because we can easily make many copies, it frees up our creativity to experiment and not care so much if we make a mistake

Choosing a Scale 

A scale drawing shows a real place with accurate sizes reduced by a factor (called the scale), to allow us to represent real objects and features on a piece of paper. Scale depends on the size of area, size of paper, amount of detail to draw. Use the equation 

Desired Scale = Width of area in centimetres/Width of paper in centimetres 

Then round the scale to the nearest 1,000. 

Changing Scale of Base Maps 

If enlarging by photocopy machine: – enlarging by 200% doubles the size of the map and halves the scale. Calculate with this equation 

 Percent enlargement = (Scale of original/Target scale) x 100%