Remote Sensing Information in Pakistan
MODIS Normalized Difference Vegetation Index (NDVI) Time Series
In monitoring crop conditions for a specific region, remotely sensed
vegetation index data are used to track the evolution of the growing season
compared to reference long-term mean conditions. A global normalized
difference vegetation index (NDVI) is produced from MODIS data, and is
referred to as the "continuity index".
A NDVI time-series database, with a spatial resolution of 250 meters has
been assembled using a 16-day compositing period, allowing for inter-annual
comparisons of growing season dynamics.
The time-series data for Pakistan are accessible through the
GLAM's web interface and analysis tool. From
this stand-alone interface the analysts can query these data by pre-defined
province/district areas, by interactive sub-setting, and by implementing crop/water
- plot time-series graphs over the crop growing seasons to quickly assess crop conditions and anomalies
- monitor current conditions
- view spatially, NDVI anomalies comparing current conditions to previous year, or historical mean
- plot histograms of current and historical NDVI data
These data and utilities are fundamental for crop yield forecasts and can serve
as an early warning system for areas suffering from crop loss and food
SPOT-VGT Normalized Difference Vegetation Index (NDVI) Time Series
This NDVI product is generated from the VEGETATION sensor onboard the
Spot 4 satellite which was
launched on March 24, 1998. It provides global coverage on an almost daily basis at
a spatial resolution of 1 kilometer.
NDVI is a measure of the amount and vigor of vegetation on the land surface
and NDVI spatial composite images are developed to more easily distinguish green
vegetation from bare soils.
NDVI is calculated from satellite imagery according to this formula:
(NIR - RED ) / (NIR + RED )
where, RED = the red portion of the electromagnetic spectrum and NIR = the near infrared portion.
For agricultural and vegetation condition monitoring, clouds are partially screened
from NDVI images by producing Maximum Value Composites (MVC) over 10-day
where the highest NDVI pixel value within the time period is retained under
the assumption that it represents the maximum vegetation "greenness" during
Physical values of NDVI range between -1.0 and 1.0 (unit less index).
The NDVI values are rescaled such that they only occupy a byte. The physical
range -0.1 to 0.92 is rescaled to the range 0 - 250, using the following formula:
Image value = (physical value + 0.1) * 250
To convert the image values back to physical values, the following formula is used:
Physical value = image value * 0.004 - 0.1
Higher values of NDVI indicate denser and healthier (higher green density) vegetation.
NDVI values of 0.1 and below, for instance, typically correspond to areas with little
to no vegetation (rocks, ice, desert). Moderate values (around 0.2 and 0.3) correspond to
shrub and grasslands and high values (0.5 and above) typically correspond to dense vegetation
like rainforests. Over the course of a growing season, we first see a steady increase
in the NDVI values as the young, green vegetation grow (the growth makes the surface
appear more and more green, which is reflected by the NDVI). This increase reaches a
maximum value just before it drops suddenly at harvest time or when the plants die naturally,
which can easily be explained by the harvesting of the healthy, green plants or their senescence,
which makes the surface appear less green.
NDVI can be very beneficial for monitoring agricultural crops. In particular time-series
are useful for estimating crop stage or growth period (i.e., planting, vegetative
development, flowering, grain-filling, etc.) and observing when periods of dryness or drought
stress occurred during the growing season.
In general, when NDVI values begin to increase, it corresponds to the start of the
growing season and the period of maximum NDVI approximately corresponds to the end
of vegetative development or the beginning of the flowering stage for most climatic
regions. It also is useful to monitor how long NDVI departures were below-average
during the growing season and if any these dry periods occurred during critical crop
stages that are especially sensitive to water stress, such as flowering or early ripening
stages, for which several dry periods during the flowering and grain-filling stages can
severely reduce potential grain yields.
NDVI is as an indicator of the amount of vegetation greenness vigor within a
pixel, positive NDVI departures from average are shaded from light green to dark green
to depict above-average vegetation conditions, and negative NDVI departures from average
are shaded from light yellow to dark brown to depict below-average vegetation conditions.
The Pakistan's Crop Information Portal contains the complete archive of
SPOT NDVI, 10-day composites from 1999 to current. Here is an example taken from
the third decade of March 2008