3.Procedure and method

3.1Match of SAR with TM
Firstly, the control point such as the cross of roads, the cross of river, the turn of rivers, lakes or pools are equably selected respectively from LANDSAT TM and topographic maps. Twelve control points are selected. The coordinates are measured respectively from LANDSAT TIM and topographic maps. LANDSAT TM image is geometrically corrected through control points. The error is controlled within one pixel.

Secondly, the control points are equably selected respectively from TM and RADARSAT SAR. Also twelve control points are selected. And then RADARSAT SAR image is matched to LANDSAT TM. The error is controlled within one pixel.

3.2Extracting the primary flood extent from SAR image.
The value differing water in SAR image from land is determined through visual interpretation. And then the model as following is used to extract flood extent from SAR image.

{DN < K is flood extent
{DN>=K is not flood extent


where, DN represents gray value in


Figure 3 The primary flood extent from SAR

SAR image, K is threshold value. When K is equal to 35, The primary, flood extent can be extracted. (Figure 3), Some shade is mislabeled as flood extent.

3.3 Extracting shade from TM image
Shade in TM can be easily identified in TM2 or TM3, because shade has low value while other has high value. The model as follow can be used to extract shade from TM image.

{TM2 < K is shade
{M2>=K is not shade
When K takes the value of 23, shade can be extracted from TM2 (figure 4).
Figure 4. The shade from TM

3.4 Removing the flood mislabeled owing to shade from primary flood extent with shade from TM image.
Figure 5 is obtained by overlaying primary flood from SAR image and shade from TM image. Gray is shade from TM, dark is primary flood from SAR image. It can be seen that shade of mountain is mislabeled as flood. The mislabeled flood overlays largely with shade from TM image. Therefore, the command EXPEND in ARC/INFO GRID is applied to expand the shade from TM. Then the shade is used to overlay with RADARSAT SAR.


Figure 5. Overlay of the primary flood from SAR and shade on TM

Overlay analysis is used to process primary flood. All floods falling into the shade from TM is removed from the primary flood extent, which is regarded as mislabeled flood. The procedure is shown as figure 6.

The result is shown as figure 7. It can be seen that the mislabeled water owing to shade is largely reduced compared with figure 3. Indeed, a few water bodies connecting with shade may be incorrectly Removed because of expanding shade. We need to correct them.
Figure 6 Removing mislabeled flood from the primary flood extent


Figure 7 The flood extent from SAR without shade

3.5 Restoring flood incorrectly removed
Firstly, base water bodies are extracted from TM image by the model as follow : {TM2+TM3) > (TM4 +TM5) – K1
{TM2>=K2
When K1 and K2 take the value 10 and respectively, base water bodies can Be extracted from TM image.