Verticals are taken with the axis of the camera pointing vertically downward. These photographs can produce more accurate maps as the variation n scale over the area is smaller and no area remains hidden. However the details in photograph can not be easily identified as the view offered is unfamiliar to eyes. Oblique are produced by giving a camera a tilt up to 30 to the

Aggregate is a granular material, such as sand, gravel, crushed stone, crushed hydraulic-cement concrete, or iron blast-furnace slag, used with a hydraulic cementing medium to produce either concrete or mortar. Those particles that are predominantly retained on the 4.75 mm (No. 4) sieve, are called coarse aggregates. Those particles passing the 9.5 mm (3/8 inch)

This is the updated description for what used to be known as 'high-alumina cement' or HAC. The problem with this cement is that it is subject to what is known as 'conversion'.

Algae are photosynthetic organisms that occur in most habitats. They vary from small, single-celled forms to complex multi-cellular forms, such as the giant helps that grow to 65 meters in length. Algae are a large group of complex-celled photosynthetic organisms.

The theodolite is the basic instrument for measuring horizontal and vertical angles. It consists essentially of a sighting tube in form of a telescope, an horizontal and vertical scales, which are so set that horizontal and vertical angles of the object sighted in the telescope can be read from the scale (see fig. 5 and 6)