Local Relief Model (LRM) Toolbox for ArcGIS 2014 David Novk

Institute of Archaeology Czech Academy of Science, Prague, v. v. i.

novak@arup.cas.cz

1

David Novk

Local Relief Model (LRM) Toolbox for ArcGIS

Guide for application of Local Relief Model in ESRI ArcGIS 10.x software using Spatial Analyst and 3D Analyst

extensions. Those two extensions have to be licensed to make the utility work properly.

UPDATE 2014-10-7: The toolbox was updated to work with ArcGIS 10.2. In old version python script was

messed up and application was unstable. Due to the changes it is needed to set the cellsize of input DEM at

the start of the process.

Download link to get the toolbox (there are both old and new version):

https://drive.google.com/folderview?id=0Bxw2O48YquY4eG5aTTloSTNWdjg&usp=sharing

List of Contents Introduction ................................................................................................................................................ 2

Software required ........................................................................................................................................ 3

How to use it ............................................................................................................................................... 3

Model visualisation ...................................................................................................................................... 5

Terms of use ................................................................................................................................................ 6

Bibliography ................................................................................................................................................ 6

Example of results ....................................................................................................................................... 7

Local Relief Model (LRM) Toolbox for ArcGIS 2014 David Novk

Institute of Archaeology Czech Academy of Science, Prague, v. v. i.

novak@arup.cas.cz

2

Introduction Visualisation of elevation data collected by aerial laser scanning (ALS/LiDAR) for archaeological use is widely

discussed topic. Every method has its advantages and disadvantages in usability, efficiency, time

consumption etc. It is obvious that simple shaded relief model (hillshade) is not sufficient for archaeological

research and there are much better ways of visualisation.1 According to J. Bofinger and R. Hess (2011),2

local relief model (LRM) is one of the most useful and efficient way to visualise elevation data stored in

raster as digital elevation model (DEM). The basic idea behind the method is filtering out terrain surface

leaving just archaeological features and their relative elevation above or below the terrain. The simplest

way to do so is to generalize DEM by low pass filter and subtract it from original DEM as it is suggested by

O. Davis in his simple guide (Davis 2012, 15-16). However, this is only the first step of the process and it will

create a very biased result, called difference model. To create fully acceptable LRM (Bofinger Hess 2011,

166-167) it is needed to:

1) Apply low pass filter3 to the original DEM -> smoothed DEM (Focal Statistics function)

2) Subtract smoothed DEM from the original DEM -> Difference model (Minus function)

3) Calculate the zero meters contours for the Difference model -> set of the break lines between the

positive and negative features (convex and concave features; Contour function)

4) Extract the real elevation of the cells from the original DEM, which intersects with the break lines -

> simplified elevation raster (Extract by Mask function)

5) Convert the simplified elevation raster to the point features (only nonzero cells) -> elevation

points (Raster to Point function)

6) Create the simplified surface DEM from the elevation points -> computed digital terrain model

(DTM; Create TIN and TIN to Raster functions).

7) Subtract the DTM from the original DEM -> LRM (Minus function)

This more complex process leads to creation of flat terrain LRM, where values express difference

between expected terrain elevation (computed DTM) and real elevation of DEM. It is extremely useful

when we using data from aerial laser scanning (ALS/LiDAR) for identification of potential archaeological

features as it is designed exactly for this purpose.

Application of this workflow is not complicated, but for common user it may be difficult to apply all the

functions referred above correctly. Creating the workflow for ArcGIS is the time consuming activity and

there is no another user-friendly tool applicable with ArcGIS, which would make this task easier. For that

reason I have created simple toolbox for ArcGIS with pre-prepared tool, which create LRM from any given

DEM raster. All used functions and tools are licensed by ESRI ArcGIS.

1 For more about DEM visualization topic and comparison of methods see Doneus 2013.

2 Described workflow is just application of Bofingers and Hesss article and the author does not intend to usurp any basic ideas of the method as his own. 3 Low pass filter is method to smooth the DEM. It goes through the data and calculates cell value as average of surrounding cells. The radius of which the mean value is computed is defined as polygon of exact dimensions. Wider polygon (or processing window) leads to smoother result and thinner polygon keeps more of the data. Setting of this value is crucial to the shape of any result.

Local Relief Model (LRM) Toolbox for ArcGIS 2014 David Novk

Institute of Archaeology Czech Academy of Science, Prague, v. v. i.

novak@arup.cas.cz

3

Software required - ESRI ArcGIS Desktop 10.0

- Spatial Analyst and 3D Analyst extensions

How to use it 1) Install the LRM toolbox by adding it to your ArcToolbox.

2) Run the LRM tool.

3) Set:

a. Input DEM

b. Cellsize of input DEM (you have to keep prefix 'CELLSIZE' in the field)

c. Low pass window shape and extent (most recommended settings are set as default)

d. Output folder

e. Click OK.

4) After all tasks are done (it may take several minutes depending on the extent and detail of input

DEM, set input values and your hardware), utility will create DTM and LRM raster dataset at your

specified output location (all intermediate data except DTM will be removed automatically). DTM

raster layer will be added to your project display automatically4 and LRM raster layer have to be

added to display manually.

5) Add LRM to your project and set desired symbology in layer properties (for example stretched; blue

colours for concave features; red colours for convex features; stretch type set to standard

deviation; n = 1).

4 IMPORTANT NOTE: If you want keep generated DTM raster, you have to move it to another location or rename it or set different output folder for next run of the utility. If you would not do that, DTM will be overwritten.

Local Relief Model (LRM) Toolbox for ArcGIS 2014 David Novk

Institute of Archaeology Czech Academy of Science, Prague, v. v. i.

novak@arup.cas.cz

4

6) It is also good point to use hillshade of same DEM with 50-70 % transparency as overlay to make

features more visible and to tune up whole view.5

5 Time to time, utility can run into unexpected error and ArcMap will shut down. In such a case just restart ArcMap and run the utility once again.

Local Relief Model (LRM) Toolbox for ArcGIS 2014 David Novk

Institute of Archaeology Czech Academy of Science, Prague, v. v. i.

novak@arup.cas.cz

5

Model visualisation

Local Relief Model (LRM) Toolbox for ArcGIS 2014 David Novk

Institute of Archaeology Czech Academy of Science, Prague, v. v. i.

novak@arup.cas.cz

6

Terms of use By downloading or using LRM Toolbox, you agree to the following terms and conditions:

Toolbox is open and free to use and modify by any user. Use, copy, share and do whatever you wish with

this software only at your own risk. The author takes no responsibility of possible damage or other

problems with your software, hardware or data caused by using the LRM Toolbox.

Bibliography

Bofinger, J. Hesse, R. 2011: As far as the laser can reach Laminar analysis of LiDAR detected structures as

a powerful instrument for archaeological heritage management in Baden-Wrttemberg, Germany. In:

Cowley (ed), Remote Sensing for Archaeological Heritage Management, EAC Occasional Paper No. 5, 163-

173.

Davis, O. 2012: Processing and Working with LiDAR Data in ArcGIS: A Practical Guide for Archaeologists.

Aberystwyth.

Doneus, M. 2013: Openness as Visualization Technique for Interpretative Mapping of Airborne Lidar

Derived Digital Terrain Models. Remote Sensing 5, 6427-6442.

Local Relief Model (LRM) Toolbox for ArcGIS 2014 David Novk

Institute of Archaeology Czech Academy of Science, Prague, v. v. i.

novak@arup.cas.cz

7

Example of results

Transect of landscape with deserted fields (1), mining objects (2) and ponds for ore processing (3-5) with

surrounding minor features.

IntroductionSoftware requiredHow to use itModel visualisationTerms of useBibliographyExample of results