What Is Topography Used For

[Louella Kammann]

Topographycombines top- with graph-, a root meaning "write" or "describe". The topography of the Sahara Desert features shifting sand dunes and dry, rocky mountains. A topographic (or topo) map not only shows the surface features of a region but also indicates the contours and approximate altitude of every location, by means of numerous curving lines, each indicating a single elevation. In other words, it shows a "three-dimensional" picture on a two-dimensional surface. Topo maps are commonly used by hikers, surveyors, government workers, and engineers, among other people.

Placido disc reflection systems measure the curvature, irregularities, tear film quality, foreign bodies, and other parts of the anterior cornea. The reflection is highly dependent on the tear film which reflects the light, and can be either small-cone or large-cone. Small cones are more accurate as they collect more data points, but large cones are easier to use and make collecting data significantly easier.

These two systems provide information about the anterior and posterior cornea, and are used for detection and management of corneal swelling, which is specifically important for contact lens wearers.

This is the most traditional way of viewing a topography image, as it is known for its overview of the corneal power. However, since it collects the averages of the data to produce a smooth map, it is considered less accurate than the other maps.

Axial maps are a helpful tool for selecting the base curve of a soft contact lens because the average of the central curvature is displayed. However, for information about corneal shape and power, other map displays are more ideal.

This map can also be used to evaluate the power of a contact lens while the lens is on the eye. This is helpful when the patient is being fitted for a multifocal contact lens and the optical powers must be correctly positioned on the eye.

This map is used to determine the true shape of the cornea and is crucial for selecting the best contact lens design for an irregular cornea. This map display is especially important when deciding between a scleral gas permeable (GP) lens or a corneal contact lens.

The elevation display map is also helpful for ortho-k management, as corneal shape is crucial in determining if ortho-k lenses will be appropriate for the patient, and in selecting a dual-axis or single-axis lens.

This map displays the quality of the natural tear film and also shows how the tear quality has been impacted by contact lens wear. A corneal topography test will measure the tear film before the patient begins to wear contact lenses and then be compared to the measurement taken after the patient has been wearing contact lenses.

A corneal topography test is quick and painless. During the test, you will sit in front of a lighted bowl that contains a pattern of rings, and rest your head against a bar. A series of data points will be collected, and a color coded image of your corneal shape will be generated on a computer screen.

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Topography is evaluated using Placido disk patterns or mires reflected off of the tear film of the anterior cornea and converted to color scales. Because the image is generated off the tear film, irregularities in tear film can significantly impact the quality and fidelity of a Placido disk topography. Secondly, lack of patient fixation may affect the quality of the topographic image. Finally, there is decreased accuracy of posterior elevation values especially after refractive surgery.

In comparison to topography which is good at capturing anterior corneal power, overall corneal shape is best captured by tomography. Scanning slit technology, Scheimpflug-based imaging, and anterior segment OCT technology is used to take multiple slit images of the cornea and provide data on both the anterior and posterior surfaces.

Scanning Slit is one of the elevation based methods for assessment of corneal curvature and power. Multiple complimentary slits are used to perform an assessment of the corneal surface (Figure 2, left). In the Orbscan , 40 slits (20 each from nasal and temporal side) are projected on the cornea to assess 240 points on each slit. The triangulation between the reference slit beam surface and the reflected beam captured by the camera can be used to analyse the anterior and posterior corneal curvature and corneal thickness (Figure 2, right).

The non planar shape of cornea can potentially lead to spurious results and therefore the use of schiempflug principle in corneal imaging is a welcome new change. Theodre Scheimpflug , an Austrian army man worked extensively on a method for correcting arial skew distortion in perspective photographs.[7][8] Even though the technique was described before him, his development of the principle led his name to be associated with the principle. In an ideal scenario , the lens plane and the image plane are parallel. Therefore a linear object will form a plane of focus parallel to the lens plane and thus can be focused totally on the image plane (Figure 3a). Consider a situation , when the object is not parallel to the prospective image plane. It will not be possible to focus all of the image on a plane parallel to image plane (Figure 3b). Thus this may lead to image distortion. However, according to the Scheimpflug principle, when a planar subject is not parallel to the image plane , an oblique tangent can be drawn from the image, object, and lens planes. This point of intersection is called the Scheimpflug Intersection (Figure 3c). A careful manipulation of the image plane and lens plane can create a focused, sharp image on the non parallel object.

Step 2 . Start by looking at the quad or multi map option as it gives the best visual comparison of the data. Below are the multimap (4x) options in the Orbscan (Bausch and Lomb, NY) topographer which is placido and scanning slit based and the Sirius (CSO, Italy) tomograph and topographer which is Placido-disk and Scheimpflug based.

Step 3. Next step is to look at the pseudocolor scale and identify the range and gradient of the values given. Each scan will have a color coding scale for the indices measured. For the absolute topographic map, these will the absolute dioptric value at that point of the cornea. As we are trained to look at patterns for keratoconus or other corneal abnormalities, a tighter color coding scale may enhance patterns or more lax color coding scale will lead to missing out patterns.

The figure 5 given below illustrates the importance of reading the grading scale and using a fixed dioptric scale which is specific for a given range. A patient with keratometric values of K1 38.5D/K2 39.5D is used as an example. As the tightness of the dioptric steps is decreased and overall range of dioptric steps in increased , the details are lost and a suspicious pattern could be overlooked. To avoid this error , most topographers use a small zone of absolute scale at a fixed interval of 0.5 D, adjusting the greenish colors to near-normal values . However , this is customizable and thus same scale should be used when comparing scans.

Step 4. On an absolute scale, the green colors are most representative of normative data and thus a quick review of the scan will help. Too much red is almost always abnormal. The scan should now be mentally compared to patterns seen in keratoconus or other suspected corneal disorder. A classical pattern may be picked up easily , however more experience is required for atypical patterns.

Step 5. The next step is to look at the actual numbers on the charts and in the statistics boxes.Numerical overlays show thinning and central corneal thickness , apical keratometry , anterior and posterior corneal elevations and specific details at a point , which can be assessed by moving the cursor on that point.The figure below shows the important numerical overlays in a Scheimpflug topography printout.

Statistics box: Useful data on SimK, minimum corneal thickness , 3,5,7 mm zone irregularity , kappa angle , pupil diameter and white to white diameter can be seen in these charts. The figure below shows the statistics box in an Orbscan topographer printout.

Step 6 . Compare with slit lamp findings again. It should always be kept in mind that corneal topography can be effected by corneal artifacts and therefore interpretative value is decreased in cases such as nebulomacular corneal opacities, dry eye , corneal neovascularisations and corneal scars.

Topography is a field of geoscience and planetary science and is concerned with local detail in general, including not only relief, but also natural, artificial, and cultural features such as roads, land boundaries, and buildings.[1] In the United States, topography often means specifically relief, even though the USGS topographic maps record not just elevation contours, but also roads, populated places, structures, land boundaries, and so on.[2]

Topography in a narrow sense involves the recording of relief or terrain, the three-dimensional quality of the surface, and the identification of specific landforms; this is also known as geomorphometry. In modern usage, this involves generation of elevation data in digital form (DEM). It is often considered to include the graphic representation of the landform on a map by a variety of cartographic relief depiction techniques, including contour lines, hypsometric tints, and relief shading.

Detailed military surveys in Britain (beginning in the late eighteenth century) were called Ordnance Surveys, and this term was used into the 20th century as generic for topographic surveys and maps.[5] The earliest scientific surveys in France were the Cassini maps after the family who produced them over four generations.[6] The term "topographic surveys" appears to be American in origin. The earliest detailed surveys in the United States were made by the "Topographical Bureau of the Army", formed during the War of 1812,[7] which became the Corps of Topographical Engineers in 1838.[8] After the work of national mapping was assumed by the U.S. Geological Survey in 1878, the term topographical remained as a general term for detailed surveys and mapping programs, and has been adopted by most other nations as standard.

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