أستخدام Raster Calculator في ArcGISمنتدى نظم المعلومات الجغرافية GIS forumهو جزء من spatial Analyst الذي يعتبر من البرامج أو الملحقات الأضافية التي تعمل مع ال
Raster Calculator يوفر العديد من الأوامر و الأدوات التي تتعامل مع المعلومات ال Raster data , اذ أن هناك العمليات الحسابية التي يمكن استخدامها من أجل الحصول على نتائج المطلوبة.
البرنامج أيضا يستخدم الأوامر الخاصة من ال Map Algebra و التي توفر الكثير من الأوامر الخاصة أذ يعتبر ال Map Algebra اللغة التي يستخدمها spatial Analyst لغة التحاليل الخاصة بها.
في ال Raster calculator يتم أستخدام العمليات الحسابية مثل الجمع و الطرح و القسمة .......الخ, و في حال أستخدام ال Map Algebra يمكن عمل العمليات الحسابية أكثر تعقيداّ على ال Raster Data.
هذا مثال على كيفية عمل موسايك Mosaic
عندي Raster Data لمنطقة سكنية فيها أرتفاعات المباني و الأرصفة و الطرق و هذه الأرتفاعات ليست مأخوذة من مستوى سطح البحر و المعلومات الأخرى هي Raster data لنفس المنطقة تم تحويلها من ال DTM و الأرتفاعات هنا من سطح البحر و العملية المطلوبة هي اضافة الأرتفاعات من DTM الى أرتفاعات المباني و الأرصفة و الشوارع......
هنا الشرح بالأنجليزية:
the Mosaic tool from Map Algebra is used in raster calculator so that the multiple input rasters can be mosaic in single raster. Mosaic uses the weighted average method that calculates the cell values in the area that are overlapping, as the input from two raster data set. In mosaic the algorithm that applied to calculate the cell values in the
overlapping areas is known as the Hermite Cubic and its formula is:
H3 (s) = 1-3s²+2s³
In the raster calculator this is how the mosaic is used to achieve the needed result as two
grids will be merging in one new raster grid with correct cell values:
([NewGrid].Mosaic ({[Grid1],[Grid2]}))
Grid1 is the result from step one, Grid2 is the result from step two and the NewGrid is the new raster data set with correct cell values.
شرح الطريقة بيانياّ
In Figure the graphical demonstration of the mosaic function is displayed.
The NewGrid will be used in assigning NoData value to some cell values which will remove some of the cell values that will not be used in the analysis, the building cell values will be removed as the will not be used in the final analysis or in the flow direction calculation. The SetNull function is used in raster calculator, the SetNull function is used in changing the cell value in the raster data
set to NoData based on the criteria that is specified by the user. The main idea from using the SetNull function is to create a mask which will mask the values that will not be used in any analysis, the SetNull setting that are used in this study are:
SetNull ([NewGrid] >9, [NewGrid])
As from the expression any cell with values that are grater then 9 will be set to NoData, as the remaining cell will keep their original values. As the NoData assigning been set up successfully the raster data set become ready to apply the GIS hydrological analysis on it, as the flow direction need to be determine and also the direction of the flow of each cell need to be done.
Figure show the raster grid before and after applying the SetNull function.
... هذه المعلومات من ضمن بحث قمت به في ال Modelling Flow with GIS
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موضوع: أستخدام Raster Calculator في ArcGIS