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Custom combination overlay for flood risk

edited February 2023 in General

Dear community,

I'm trying to make a custom combination overlay between two layers from the Rainfall overlay: Surface Max Speed (V) and Surface Max Value (D). I have used these output overlays before in a project to create a risk map based on the flood severity levels from Smith, Davey & Cox (2014) [1]. In ArcGIS I'm able to make this overlay in the Raster Calculator with the following SQL:

Con(("V"*"D" <=0.3) & ("D" < 0.3) & ("V" < 2.0), 1,

Con(("V"*"D" <=0.6) & ("D" < 0.5) & ("V" < 2.0), 2,

Con(("V"*"D" <=0.6) & ("D" < 1.2) & ("V" < 2.0), 3,

Con(("V"*"D" <=1.0) & ("D" < 2.0) & ("V" < 2.0), 4,

Con(("V"*"D" <=4.0) & ("D" < 4.0) & ("V" < 4.0), 5,6)))))

In Tygron I should be able to do this as well in a custom combination overlay, but I'm not that experienced yet with TQL and keep getting invalid arguments. Could someone have a look at what might be going wrong?:

IF(LTE(MULT(A,B),0.3) AND (LT(B,0.3)) AND (LT(A,2.0)),1,

IF(LTE(MULT(A,B),0.6) AND (LT(B,0.5)) AND (LT(A,2.0)),2,

IF(LTE(MULT(A,B),0.6) AND (LT(B,1.2)) AND (LT(A,2.0)),3,

IF(LTE(MULT(A,B),1.0) AND (LT(B,2.0)) AND (LT(A,2.0)),4,

IF(LTE(MULT(A,B),4.0) AND (LT(B,4.0)) AND (LT(A,4.0)),5,6)))))

(where A = Surface Max Speed , and B = Surface Max Value)

[1] Smith, G. P., Davey, E. K., and Cox, R. J. (2014). Flood Hazard Water Research Laboratory Technical Report 2014/07. Prepared by the Water Research Laboratory. April 2014. Accessed on 23/02/2023 via


  • Hi there!

    As a quick remark to steer away from confusing terminology: The Combo Overlay itself does not use TQL. The Combo Overlay uses a collection of mathematical and logical functions to perform a computation. More information about those functions can be found here:

    In the Combo Overlay, all functions take on the following format:


    At a glance, the "AND" sections of your formulas are the most obvious requiring a change. Rather than using "A AND B", write it as "AND(A,B)".

    This means that the following formula fragment:

    IF(LTE(MULT(A,B),0.3) AND (LT(B,0.3)) AND (LT(A,2.0)),1, the-rest)

    Should be rewritten as follows:

    IF( AND( LTE(MULT(A,B),0.3) , LT(B,0.3) , LT(A,2.0) ),1, the-rest)

    (spaces added for an attempt at legibility)

    Can you try this and see whether this works for you?


    Sprawling spreadsheets so intricate Alexander the Great cuts them in half.

  • Hi Rudolf,

    Thank you very much! Yes, it now works and looks as follows:

    IF(AND(LTE(MUL(A, B), 0.3), LT(B, 0.3), LT(A, 2.0)), 1, 

    IF(AND(LTE(MUL(A, B), 0.6), LT(B, 0.5), LT(A, 2.0)), 2, 

    IF(AND(LTE(MUL(A, B), 0.6), LT(B, 1.2), LT(A, 2.0)), 3,

    IF(AND(LTE(MUL(A, B), 1.0), LT(B, 2.0), LT(A, 2.0)), 4, 

    IF(AND(LTE(MUL(A, B), 4.0), LT(B, 4.0), LT(A, 4.0)), 5, 6)))))

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