week 7 THE MODEL BUILDER

Transcription

1 week 7 THE MODEL BUILDER

2 topics of the week What is a model Computer simulation models Model Builder in ArcGIS Model Builder s interface Using the Model Builder

3 What is a model? A model is an approximate substitute for a real system. It is a human construct to help us better understand world systems. Example of a model building plans Key features in the development of a model a goal must be clear initial criteria must be identified for reaching that goal simplifying assumptions must be made the range of applicability of the model should be understood limitations of data/parameters must be known

4 Computer simulation models A special category of models computer simulation models calculations are turned over to computers the logic of the model s construct remains with us the flow of the processes can be automated not always they facilitate experimenting they facilitate re-runs they are easier to be modified, tweaked, expanded they are easier to be shared and examined they facilitate documenting processes and parameters they increase our understanding of the system

5 Main elements of a computer model Data Input data Intermediate data Output data Overall process flow, or logic of the model, or step by step operations Functions/algorithms implemented during the process (buffer a road, or select parcels) Parameters (100 m buffer, or parcels < 5 acres)

6 Example from our Suitable Location Exercise 8 Parcels Select Vacant Residential Vacant Parcels Select Parcels Containd within the block Target Parcels Select P >= 25 Census Blocks Targets Major Roads Select CLASS = Interstate or US Roads Highways Buffer 1/2km Select Parcels within buffer Highway Buffer Scenic Roads Buffer 1km Scenic Buffer Select Parcels within buffer Final Parcels

7 Limitations of models They represent the real world i.e. are inherently inexact. The input data can be inaccurate. The logic of the process flow can be imperfect. The individual functions used may be inappropriate. The parameters used may be uncertain as they are usually determined empirically. Any or all of the above.

8 What is Model Builder Model Builder is an environment in ArcGIS that allows for constructing a GIS based computer model. Other GIS software may have other environments that provide similar capability. Model Builder allows for a sequence of geoprocessing operations to execute automatically, and to generate a final dataset output. It allows for modification/change of all the model s elements i.e. the model can be refined an re-run to produce alternative outcomes.

9 The Model Builder Interface ModelBuilder s interface is a graphical environment for creating and interacting with the model. The white empty background is called canvas. The main building block of a model is called a process. A process consists of a geoprocessing tool. The individual parts are called elements. All elements, when tied together with connectors, make up the model diagram.

10 The Model Builder Interface The model diagram shows all the processes and the sequence in which they run. The connecting arrows show how elements and processes are related to each other.

11 The Model Builder Interface Basic idea is that of a dependency diagram User specifies inputs, processing, and outputs If inputs change, system repeats intermediate operations with new inputs The diagram has three kinds of elements Inputs Geoprocessing operations Outputs Output from one operation can be used as input to the next, allowing for chaining Connector arrows show direction of processing.

12 Using the Model Builder Open ArcToolbox. Right-click in a blank area in ArcToolbox Choose Add Toolbox Right Click on the new Toolbox (FireTools) Click New > Model

13 Using the Model Builder Before starting your model, save it, name it, and set any of the appropriate properties.

14 Using the Model Builder To build a model, you drag tools from the appropriate toolbox into the model. Then click on the tool (such as Buffer), and fill in the parameters. Drag layers from the table of content. Finally, connect data and processing boxes together. When you are done the elements turn blue, yellow, or green.

15 Model Builder s interface Model elements can be in one of three states: blank or white elements are not ready to run colored elements are ready to run colored elements with a dropshadow have finished running

16 Storing and retrieving models Models are saved in an Toolbox (new or existing) in a.tbx format. Models are saved in a Toolbox by default under: c:\users\username\documents\arcgis New models can be added to the Toolbox from within a project, but they reside on disk, outside of the project. The Toolbox can be removed from the project and added later, or it can be saved within the project. Models can be saved within.mxd projects in ArcToolbox or they can be deleted from.mxd projects. They relate to.mxd projects similarly to datasets.

17 Generalizing models A few words about generalizing Models By default, models only use exact data as originally specified To convert a model into a true tool we need to specify which inputs / outputs are variable parameters Right mouse on input or output Select Parameter (toggle) After Parameters are set, double clicking brings up user dialog