Change Detection Steps Remote Sensing System and Environmental Considerations

Remote Sensing System Considerations

 Temporal Resolution

• Data Should be Aquired the Same Time of Day

The data should be obtained from a sensor system that acquires data at approximately the same time of day. Most Landsat Thematic Mapper data is acquired before 9:45 am for most of the conterminous United States. This eliminates diurnal sun angle effects which can cause anomalous differences in the reflectance properties of the remotely sensed data.
• Data Should be Collected on Anniversary Dates

Data collected on the same day in different years removes seasonal sun angle and plant phenological differences which can destroy a change detection project.

 Spatial Resolution and Look Angle

• Data Collected with the Same Instantaneous-Field-Of-View (IFOV)

When remotely sensed data are acquired by a sensor system at the same IFOV, registration is much easier and more accurate. If IFOV is different between the two images, resample to a uniform pixel size.
• Rectification Root Mean Square Error (RMSE) of < 0.5 of a pixel

Misregistration between the two images may result in the identification of spurious areas of change between the data sets.
• Data Collected from the Same Look Angle

Two images with significantly different look angles can cause problems when used for change detection purposes. Differences in reflectance from the two datasets may cause spurious change detection results.

 Spectral Resolution

• Data Should be Collected From the Same Sensor in the Same Bands

Data aquired on multiple dates from the same sensor system in the same bands achieves best results. When this is not possible, bands which most closely approximate one another should be used.

 Radiometric Resolution

• Data Should be Collected at the Same Radiometric Precision on Both Dates.

When the radiometric resolution of data acquired by one system are compared with data acquired by a higher radiometric resolution instrument (e.g., Landsat TM with 8-bit data) then the lower resolution data (e.g., 6-bit) should be decompressed to 8-bits for change detection purposes. However, the precision of decompressed brightness values can never be better than the original, uncompressed data.

Environmental Considerations

 Atmospheric Conditions

Ideal atmospheric conditions for the collection of remotely sensed data include no clouds, stratus, or extreme humidity. Cloud cover greater than 20% is usually considered unacceptable. Analysts must closely consider the amount of acceptable cloud cover in their study area. Clouds not only obscure terrain but the cloud shadow may also cause major problems in image classification.

If dramatic differences exist in the atmospheric conditions present on the n dates of imagery to be used in the change detection process, it may be necessary to remove the atmospheric attenuation in the imagery. If substantial in situ data are available on the day of the overflights, sophisticated atmospheric transmission models or alternative empirical methods may be used to correct the remote sensor data from dramatic atmospheric differences (Jensen 1996).

 Soil Moisture Conditions

Under ideal conditions, soil moisture conditions should be identical for each date of imagery used in a change detection project. Changes in wet or dry conditions on one of the dates can cause serious problems in the change detection process. Problems caused by changes in soil moisture conditions may be avoided by carefully reviewing precipitation records to determine how much rain or snow fell in the days and weeks prior to remote sensing data collection. When soil moisture differences between dates are significant for only certain parts of the study area (perhaps due to a local thunderstorm), it may be necessary to stratify (cut-out) those affected areas and perform a separate analysis which can be added back in the final stages of the project.