Digital Image Processing: An Introduction

Digital Image Processing (DIP) is a technique which involves manipulation of digital image to extract information. When satellite images are being manipulated in such manner, this technique is also referred to as satellite image processing.

It involves combination of software-based image processing tools. The whole process of Digital Image Processing can be classified into three parts-

Image Pre-Processing

The raw satellite images may contain variety of errors in their geometry and radiometry (cosmetic appearance). Hence it is important to rectify these images before starting their interpretation. This typically involves the initial processing of raw satellite image for correcting geometric distortions, radiometric corrections & calibration and noise removal from the data. This process is also referred as Image Rectification. Image pre-processing is done before enhancement, manipulation, interpretation and classification of satellite images hence it is called so.

Image Enhancement

Before starting visually interpreting satellite images, some image enhancement techniques are applied to manipulate theme to improve and enhance features. This helps in better interpretation of the images and in segregating one feature type to the other. Image enhancement involves use of a number of statistical and image manipulation functions provided in image processing software. These include contrast enhancement, histogram equalization, density slicing, spatial filtering, image ratio (like RVI, NDVI, TVI etc.), principal components analysis (PCA), colour transformations, image fusion, image stacking etc.

Image classification

It is software–based image classification technique which involves automated information extraction and subsequent classification of multispectral satellite images. These are statistical decision rules which groups pixels in different feature classes. Digital classification techniques are less time consuming than visual techniques. Digital satellite images can be classified digitally using supervised, unsupervised or hybrid type of image classification (these will be discussed in detail in a separate blog).

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Things to Remember while Starting GPS Survey

Before leaving for field work always check whether your GPS instrument is working properly.

If there are waypoints and tracks are stored in the GPS instrument during previous survey then it is advisable to download them on your computer system and then erase them from the GPS. In this manner you will have maximum space available to store waypoints etc. for the new survey.

Always carry user manual and technical help guides of your GPS instrument with you.

Always ensure to carry an extra pair fully charged rechargeable batteries (and of course don’t forget to carry battery charger!).

Please do not start survey immediately after switching on the instrument. Wait till the GPS starts displaying x & y positions and positional accuracy in stable manner (i.e. till these values stops fluctuating rigorously).

Always check the page which displays signal strength and number of satellites available.

During GPS survey avoid recording points below trees, high rise buildings and other obstructions as they weakens the signal strength hence decrease the accuracy. Some times such obstructions in may break the signals completely.

If you are recording waypoints/tracks while navigating in a vehicle it is advisable to use external antenna connected to your GPS (rather than carrying the instrument in your hand and keeping it outside the window of the vehicle which is not only dangerous but tiring also).

At the end of the day always ensure for availability of sufficient space in the instrument for the storing waypoints/tracks for the next days’ survey.

Although most of GPS instruments are rugged, try to avoid any kind of mishandling and carelessness.

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Selecting Right GPS

Selection of GPS is solely depends on user requirement and of course, availability of funds. There are numerous models of GPS available in market and there prices vary according to functions, features and accuracy of measurement.

Most of the basic handheld GPS provides facility to store limited number of waypoints (for example a Garmin60 handheld GPS can store up to 512 waypoints), tracks and routes. Most of these instruments display information like vehicle speed, distance traveled, positions of satellites, quality of satellite signals, direction of North, coordinates (x,y and z) and time. Apart from all these features, one must also ensure following important things before purchasing a GPS instrument-

User-friendly- A good GPS instruments should be user-friendly so that anybody should be able to operate, at least its basic functions, with ease. For example- One should be able to record a waypoint just by 2-3 clicks of keys.

Interface- As now a days USB ports become universally used interface, one should ensure that the GPS instrument must have provisions for both COM port and USB connectivity.

Ruggedness- GPS is completely field-based outdoor instrument and it is widely-used in variety of field condition like tough terrains, various climatic conditions (extreme summers, winters, rains etc.) and dust. One should always ensure that it should be rugged enough to operate properly in such conditions.

Accuracy- Most of the handheld GPS provides positional accuracy in the range of 5-15 m. The cost of instrument increases with the enhancement in the accuracy.

Troubleshooting & Technical Assistance- Before purchasing GPS instrument of a particular brand one must ensure that technical executives of that GPS company are available and accessible with ease to you so that, incase, the instrument doesn’t work properly one can contact them immediately to save valuable time on field.

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