Techniques for Area Measurements: Dot Grids and Line Intercept or Transect Method (Especially for GATE-Geospatial 2022)

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  • The important thing to keep in mind, once we mastered measuring distances, is that areas have squared units. For a rectangular area its (length) × (width) , so if we measure both and convert these distances remember that if we are multiplying them together the resulting units are squared.
  • For example, if an area is 100 meters by 500 meters, it is 50,000 square meters. The precision of a measurement is dependent upon; ability to determine photo scale, the precision of the conversion factors, the precision of the measuring device (e. g. using a standard 12-inch ruler or a millimetre-scaled ruler) and the accuracy with which we can determine the edges of a feature or area.
Area Measurement

Polar Planimeters

Polar planimeters, also known as a platometer, is a measuring instrument used to determine the area of an arbitrary two-dimensional shape. It is used on traditional maps.

Line Intercept or Transect Method

  • Line intercept or transect method of canopy estimation is analogous to the dot grid method and is similarly accurate. In this method, lines are superimposed on the aerial image and the length of each line that overlays tree canopy is compared to the total line length. Canopy cover is then calculated as % canopy cover = 100 x (length of lines covered by trees/total length of lines in the sample) . Lines may be printed on a transparent sheet or can be designated by randomly dropping a clear scale on the photo.
  • If streets or other features are arranged in parallel lines, sampling bias is best avoided by using a random arrangement of lines rather than parallel lines on the sampling overlay. Accuracy is improved by using more short lines rather than a few long lines.

Dot Grids

  • Dot grid area estimations involve laying a transparent grid over an area of interest and counting the grid cells or dots that fall within that area. This is a quick and easy way to estimate areas, or to estimate the density of objects, and is relatively easy to understand.
  • Each dot or grid cell is proportional to an area according to the scale of the source image, summing the number of dots or grid cells and multiplying by the scale conversion allows us to estimate areas quickly.

Area on Dot Grid

The below diagram illustrates method for estimating area. The scale is fixed at 1 km. This method has tradeoffs between precision and accuracy but is a valid method of estimating areas.

Dot Grid Method

Dot Grid Method

Dot Grid Method of Canopy Estimation

This is an easy, accurate, and relatively rapid method for determining canopy cover, and is equally applicable to natural woodlands and planted urban forests. A dot grid is a sheet of transparent material imprinted with dots arranged in a regular grid. The canopy cover estimate is made by laying the dot grid over the area of the aerial photo to be sampled and counting the number of dots that fall on tree crowns. Per cent crown cover can then be calculated as:% canopy cover = 100 x (dots falling on trees/total number of dots within the sampled area) .

  • Sampling Bias: may be a problem if a regular dot grid is superimposed on a photo with features that repeat in a regular pattern, such as rectangular city blocks in which case make sure that the dot grid is always skewed relative to the street grid to minimize sampling bias.
  • Sample Size: How many dots do we need to count? Unfortunately, there is no single answer to this question, but we must calculate the minimum sample size of dots required for a given application. Several basic principles apply when determining the necessary sample size. The reliability of the canopy cover estimate will increase as the dot density increases, but the increase in statistical power begins to plateau at high sample sizes.
Plateau High Sample Sizes

Example Photo on 1: 20,000 Scale, Each Square Contains 25 Dots and is 1 Cm on a Side. How Big is this Lake?

The graphic (above image) is an example of the dot grid overlay method. All of the necessary information is given. Remember that these are area units so they are squared and that the scale given is a linear scale (not are area scale) . Below figure is a standard dot grid. Note that if this dot grid is printed that the darker lines are intended to be one inch apart, this graphic needs to resize, if it is to be printed on the transparencies.

Graphic Dot Grid Overlay Method

Scale a grid cell relative to the photo being measured (real world area) and divide by 4 (how much area each dot represents) . This graphic need to resize, if it is to be printed on the transparencies.

Stereoplotters for Calculating Area and Lengths

With the techniques described above, it will be possible to make estimates of areas and lengths. This can be useful when interpreting air photos because sometimes relative sizes, and differences in areas, can lend support to an interpretation. When exacting measurements are required however dot grids and scaled measurements need an additional level of correction. The devices for performing this correction are called “stereoplotters” . There are two types:

  • Analogue stereo plotters require specialized knowledge and calibration but yield extremely accurate measurements when used correctly in conjunction with enough geometric control.
  • The second type of device is called an “analytical stereoplotter” and is digital. The main benefit of these devices is that they are usable by trained individuals and are very reliable when maintained properly.
Extract Resource Elements - MATLAB lteExtractResources

Geographic Information Systems (GIS) as well as most image processing software packages have image registration capabilities that have replaced manual area estimation techniques. But not in all cases is GIS feasible or practical, and if we lack enough ground control points to geo-rectify the imagery the additional cost of doing so may not afford enough accuracy to be cost effective. Image registration will be covered in another module, but suffice it to say that the manual techniques discussed so far will provide reasonably accurate inexpensive estimates of area and length.

Dot Grid Area

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