I have two sets of data. The one is daily temperature data and another is 16-day vegetation observation data (value of Jan 1, Jan 17 etc), throughout a year. But I need daily values of vegetation observation data. How to obtain daily value from 16-day vegetation data?
You might obtain estimates of vegetation observation data by interpolating the data you have. You can choose among the different interpolation methods, linear, polynomial, spline etc.
In the figure below the data you provided are plotted together with a linear interpolation (green line) and a third order polynomial interpolation (red line). Note that with the polynomial interpolation you can get some weird effect, such as negative vegetation observation data points between the first two dates.
And here are all the values you would get by interpolating the data you provided. In the second column are listed the outputs of the linear interpolation, in the third column, the data of the polynomial interpolation.