Selecting contours which heights are integers?

Question

I have a Contours Featureclass, with an interval of 0.2 meters. I wish to select only those Contours, which are at 1 m interval?

I have tried using the following syntax in the 'Select by Attribute' dialog (in ArcMap), but it selects all the contours:

Mod("ELEVATION", 1)=0

What query should I run?

Saved a LOT of time for me, really thanks! Never seen this query codes like "Flor", "Elevation" and "Mod". — , Oct 28 '15 at 12:51

Devdatta Tengshe · Accepted Answer · 2013-08-23T09:01:41.417

27

The simple trick to select only Integer values, is to use the following syntax:

Mod(Round("ELEVATION", 0)*10, 10)=0

This Multiplication by 10, makes all the values Integer, and then we select only those which are multiples of 10.

In case you wish to select Multiples of some other number, just multiply 10 by the interval.

To get contours at 5 m intervals, use:

Mod(Round("ELEVATION",0) * 10, 50)=0
To get contours at 100 m interval use:

Mod(Round("ELEVATION", 0) * 10, 1000)=0

Update
As per Whuber's advice given in the comment below, I have added the rounding function in the query expression.

edited Aug 23 '13 at 09:01

answered Apr 17 '13 at 05:12

Devdatta Tengshe

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7

I don't think I would trust these solutions. The problem is that with non-integral intervals of 0.2, the database is likely storing them as floats. Consequently MOD will be subject to floating-point roundoff error--and that's critical here: if you're low by even the very least significant digit, MOD may return the wrong value (depending on how it is implemented). I would like to suggest rounding the values before applying MOD as a way of preventing these subtle (and insidious) errors. – whuber Apr 17 '13 at 16:08
You need to round after multiplying by 10, but before finding the mod. – smithkm Aug 23 '13 at 15:59
1

If I want to select contours at 5m intervals, the formula implemented as-written above selects non-integers that round to a number that is evenly divisible by 5. E.g., 14.5m and 19.5m were selected but this is not desired. These were not selected if I dropped the 'Round' part of the function. – delongtime Jan 06 '14 at 17:50
1

@delongtime If you have contours that are supposed to have non-integral levels, simply modify the second argument of ROUND to achieve more precision in the rounding. – whuber Jan 06 '14 at 17:58
I am not sure the update of this was done correctly. Shouldn't the Round be done on the "ELEVATION" * 10 not the "ELEVATION". Maybe like this: Mod(Round( "ELEVATION" *10 , 0), 10)=0 – Rex Dec 09 '20 at 18:53

score 9 · Answer 2 · edited Aug 04 '14 at 02:18

Here is another query option. It largely does the same as the answers listed above but is (in my opinion) a little bit easier to customise for different scenarios.

To display any contour divisible by 10

Floor(Elevation/10)=Elevation/10

To display any contour divisible by 50

Floor(Elevation/50)=Elevation/50

To display half metre contours

Floor(Elevation/0.5)=Elevation/0.5

To display 10 metre contours with an arbitary offset (eg 10.2, 20.2, 30.2)

Floor((Elevation-0.2)/10)=(Elevation-0.2)/10

At the end of the day, this is just another option to consider.

user23715 · Answer 3 · 2014-04-10T20:55:24.690

This was meant to be a comment on the above answer - sorry.

Syntax would vary depending on the type of DB your Contours are stored in but the given solution seems to pick contours after rounding their values. So for example in my test this achieved a selected set that included .3 m as well as .4 m. In fact, it excluded none of the values.

This equation

Mod(Round("ELEVATION" * 10, 0), 2)=0

Gave me results that seemed to match what the questioner was asking.

score 1 · Answer 4 · answered Feb 02 '16 at 17:59

To get Index contours I use this on Field Calculator:

In ARCMAP

dim dIndexInterval
dim dCont
dim i
dim j
dim k
dim c

dIndexInterval = 200 ' set to interval of index contours
dCont = [level] ' Set to contour height field

i = ROUND(dCont, 0) * 10
j = dIndexInterval * 10
k = i MOD j

if k = 0 then
 c = 1 ' is an index contour
else
 c = 0 ' is not an index contour
end if

'Type "c" (without quotes) in the text input field at the bottom of the dialog box and click OK.

In QGIS

if(((round("level", 0) * 10) % (200 * 10)) = 0, 1, 0)

""level"" Set to contour height field "200" Set to interval of index contours

Then: "1" is an index contour and "0" is not an index contour

score 0 · Answer 5 · answered Jan 26 '17 at 17:16

Multiply the contour by 10 and then modulus by 10 to determine if there's a remainder. If there's no remainder than it's an integer, else it's float.

def contour_class(contour):
    if ((contour * 10) % 10) == 0:
        return "Integer"
    else:
        return "Float"

__esri_field_calculator_splitter__
contour_class(!Contour!)

Selecting contours which heights are integers?

5 Answers5

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