Tag Archives: raster

How to extract the values from Time series data in Idrisi

15 Apr

Hello guys,

Time series data

With GIS you can explore a series of rasters depicting  the same geographical area through time, analyzing how an specific element or attribute changes. This is one of the purposes of Time series data. So, the other day I exchange a series of interesting emails with Idham Khalil from the Geography Department here at uni about how to extract and compare the values from a series of images.

If you have only few images, one way to compare two rasters is to subtract  one image from the other to see the changes through time, but of course it depends on the question you need to solve. On the other hand, if you have plenty images here is the solution to which Idham arrived using Idrisi Taiga:

Let say you are working with time series data and have hundreds of images(layers). And you want to find the behaviour of one pixel/polygon over times. One way of doing it is using earth Trend Modeler module in Idrisi Taiga. Load all the images in earth trend modeler and make them a raster group.Then go to Explore Temporal Profiles. This will generate a time series curve of certain pixel/region depending on your objective. Right click on the graph produced and …copy to clickboard as text(data only). Bingo….you will have the values of all those years.

As always, I deeply enjoy these conversations with other people so if you have something in mind that we can discuss regarding technical or theoretical issues behind GIS, please feel free to contact me 🙂

Happy modelling!

FREE Aerial photography and remote sensing data

1 Mar

Hey guys,

I hope you had a good weekend. These past days I have been a little bit quiet but is for a good reason. I have been doing a very entertaining research on sources for aerial photography. So, for my friends that do research in the amazing Andalucia, you are probably aware of this site which is from the Spanish government and that allows you to download all the orthorectified aerial series, from the 1956 flight and the 2007 infrared to the colour 2009, some of them at 0.5m resolution!  To tell you the true I’m very impressed with the efficiency of the guys from the Junta de Andalucia as they not only have all the material free and available, but they  also have loads of environmental information online and free as well. All cartography units should follow their schema…

Anyway, I also found this NASA site where you can download some of the Landsat series also for free. Just be aware that it doesn’t work with Chrome :p

Well, see you soon!

How to reclassify raster with batch file in Idrisi

30 Nov

Ok guys, today I had to reclassify a raster to create new ones with the values I needed. They were around 226 so I needed to make it an automatic process.  To do this in Idrisi you can create a batch file or macro. Although the process is fairly simple there are always good tips that can save you loads of time.

The way to start is to know the structure of module you want to reproduce in the macro. For “reclass” in idrisi you need the next parameters:

1.  x (to indicate that you are using macro mode)

2. File type (i for image, v for vector or a for attribute value file)

3. the input filename

4. output filename

5. classification type (1 for equal interval, 2 for user defined and 3 for file mode)

In my case, because I wanted to do my own classification I used type 2 in which you can repeat steps 6 to 8:

6. new value to be assigned to the old ones

7. old value (ranging from…)

8. old value (to those just less than…)

9. -9999 to indicate the end of the sequence

10 type of output data, 1 for integer and 2 for real.

To create the macro I use excel inputting these parameters to each cell in the datasheet:

Because you need to transform this into a macro file in Idrisi you have to save it as text. Any kind of text I have tried to save it directly from excel mess up the result so I found out that if better to do this:

I saved it as .CVS comma delimited, I opened it in word and did find and replace commas for space and then saved it as plain text.

Finally, you just open it with Edit in Idrisi save it as .IML and run the macro as usual!

I hope this is of some help because when you have 1,000,000 files to reclass it can be really handy!

Thanks to Boyd Zapatka in Clark Labs for his help!!! 🙂

How to import an Arcgis raster to Idrisi considering the “NoData” values

19 Nov

Hi again! The conversion between Arcgis rasters such as grids and others to the Idrisi format is fairly simple. However, when you run some of the analyses in Idrisi such as Varcost or to use the Land Change Modeller, you will realize soon that Idrisi gives you an error such as “floating point overflow” and others. This seems to occur when one or more of the rasters you are using have NoData values that are extremely large such as -9999.

The way to solve it is to reclassify your raster either in Arcgis or Idrisi:

1. In arcgis with the reclassification tool delete all the entries leaving only the “NoData” value. Give these values a new value of -1 (for instance)

2. If you have your file already in Idrisi, run reclassify and if you have for instance values of -9999 where in arcgis was NoData, you just Assign a new value of -1 To all values from -9999 To just less than 0.

I hope this is useful for somebody because it took me a while to find the solution to this 🙂

Have a great day!

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