An aspect map that shows the direction of slope of the landscape is an essential input in modeling the

flow direction of a stream. This paper’s objective is to illustrate a novel method of comparing and quantifying the difference between two aspect maps created using different algorithms from two industry leading GIS softwares Idrisi and ArcGIS but same elevation data at different resolutions. The paper demonstrates that at high resolution (30m), the mean of difference between the aspect maps (MDA)from the two softwares is 250 but as we coarsen the resolution(30,720 m), MDA decreases to 30.Thus, we can conclude that the flow models produced by using the aspect maps created using Idrisi andArcGIS will show a bigger difference when using high resolution data when compared to flow models produced by using the low resolution aspect maps.

1

Elevation

Aspect ArcGIS

Slope IdrisiSlope ArcGIS

Aspect Idrisi

Sin ArcGIS

Cos ArcGIS

Sin Idrisi

Cos Idrisi

Coarser AspectArcGIS

Coarser AspectIdrisi

Scatter Plots(Coarse)

Scatter Plot(Fine)

Vec

tor

Ad

dit

ion

to

Co

arse

n P

ixel

s Resultant Vector (a+b+c+d) =

Sin θa + Sin θb + Sin θc + Sin θd Tan -1

Cos θa + Cos θb + Cos θc+ Cos θd

The results show that the aspect maps produced by the two algorithms in 30m resolution have MDA of 250. As we coarsen the resolution to 30,720 m, MDA comes down to 30. Thus, the two algorithms produce different aspect maps at fine resolution (depicted by a high MDA) but the differences become smaller as the resolution becomes coarser. Therefore, this paper shows a new method that:

Compares directional maps Compares the directional maps at different scales

*MDA = Mean of difference between the aspect maps

2

30 m 120 m 480 m 1920 m 30720 m7680 m

Scatter Plots : X Axis: ArcGIS Aspect Values Y Axis: Idrisi Aspect ValuesX ranges from 0 to 360 degrees & Y ranges from 0 to 360 degrees

Ap

ply

ing

Slo

pe

Wei

gh

tag

e

On the left, the Idrisi pixel is facing West with a slope of 10 and the ArcGIS pixel is facing East with a slope of 20, on the right, the Idrisi pixel is facing West with a slope of 450 and the ArcGIS pixel is facing East with a slope of 700. The pixel values on the right will be more important than that to the left. Slope acts as magnitude in vector addition for coarsening the pixels.

Circular Variable : More than one ways of reaching between A & B

Aspect is a Circular Variable

If the value of a pixel in Idrisi map is 3500 (A)and the value of a pixel in ArcGIS map is 100 (B)then the difference between the two pixels can be both 3400(Anticlockwise) and 200(Clockwise). We used the least difference (200) :

Xn = Value of a pixel in ArcGIS aspect map Yn = Value of a pixel in Idrisi aspect map in Dn = Least difference in angle between Xn and Yn

If Xn – Yn <= 180 Then Dn = Xn – Yn Else Dn = 360 – (Xn – Yn)EndIf

WW EESlope 10 Slope 20

WW EESlope 450 Slope 700

WW EESlope 10 Slope 20

WW EESlope 450 Slope 700