The prescription map can be generated based on the orthomosaic with a calculated index (NDVI). You can calculate NDVI using Set Raster Transform option from the Tools menu (Tools > Set Raster Transform...) to open Raster Calculator dialog. On the Transform tab specify the formula for the index that you would like to calculate from the source data:

  


Select Generate Prescription Map command on the Orthomosaic submenu from the Tools menu (Tools > Orthomosaic > Generate Prescription Map...). The dialog window displays the index histogram and the division into zones. Also, a separate table shows the value of Zones, Area (ha), and Rate (kg/ha). These values in the table can be changed as well as the Total amount (kg).




When opening the dialog box, wait for the histogram calculation in the dialog box:



In Metashape Prescription maps are built according to the following algorithm: 

Specify the area along with the boundary vector layer (if there are no vector layers, orthomosaic is used in its entirety). Using Polygon tools for drawing areas for prescription maps.



The selected area will be transformed if there is an active transformation rule (Raster transform). For the resulting raster, a histogram of the values of the first transformed channel is built and this histogram is divided into clusters. 


In Metashape available 2 methods for clusterization: Jenks natural breaks and Equal intervals clustering: 

  • Jenks natural breaks. When using the Jenks natural breaks clustering method, the splitting occurs automatically. 


Result:



  • Equal intervals clustering. When using the Equal intervals clustering method, the histogram is split into equal intervals in the interval from the min index value to the max index value. Min and max value index calculated automatically:




Result:

 


After this step, the cells that fall into the same cluster are combined and for each cluster, its borders are build and written into a vector layer named Prescription. The resulting layer contains several groups of polygons (MultiPoygon) corresponding to the clusters. The attributes of each group contain the average value of the index for the given cluster (zonal_mean) and the ordinal number of the cluster (cluster_id), starting from 1. To open Properties tab select View > Properties command. If you select shape on the Ortho view, you can see Properties on the Properties tab:



In our case we select 4 clusters: 




To change the fertilizer consumption, after building the Prescription map layer, select Prescription map on the Workspace pane and choose Prescription map properties... command on the context menu:



You can export Prescription map in Shapefile format, select Export Layers... command on the Prescription map layer on the Workspace pane, and select this format.