Where Does Data in AutoCAD Drawings Come From?
Today we’re featuring a guest post from Tripp Corbin of Keck & Wood. Keck & Wood is a GeoSpatial Training Services business partner, and Tripp and his team created the instructor led and e-learning course “Using AutoCAD Data in ArcGIS“. This course explains the .dwg format and how it can be integrated into standard ESRI data formats such as shapefiles and geodatabases.
Like GIS data, AutoCAD data can be created in a number of ways and accuracies. There is a general assumption that data created by Engineers and surveyors is always extremely accurate. This is not always the case. The accuracy will depend on the methods used to collect the information found in the drawing. The most common methods for data collection include:
• Land Surveys (Conventional, GPS, Station & Offset)
• Using Existing Drawings
• Importing from a GIS
• Digitizing (Tablet and Heads-up)
Each of these methods have varying degrees of accuracy. As with any data you might want to integrate into your GIS, it is import to know how the drawing you are working with was created. Let’s look more closely at some of these methods engineers use to collect data.
Land Surveys are one of the most common ways data is collected for AutoCAD drawings. Engineers and other professionals use land surveys because they capture the current real world conditions for the area related to their project. There are many methods that can be used by Surveyors to collect data. The three basic types of surveys are:
1. Conventional (Transit, Levels, Total Stations)
3. Station and Offset
Each of these methods have different levels of accuracy as well as pros and cons.
Conventional land surveys are what most people think of when you mention surveying. This is the person standing on the side of the road using an instrument such as a transit, level or Electronic Total Station (see picture above). Such surveys generally produce very accurate relative data. This means the distances and angles between the various features collected are extremely accurate. However this information is not normally tied to a specific geographic location. Instead it is collected in a local or assumed coordinate system.
It is not impossible to tie a survey done with conventional means to a true geographic location. If a control point or monument is near by a surveyor can use it to georeference his survey data. Many locations have laws that will actually require surveyors to do this. The problem is that there are not normally any control points or monuments near by for surveyors to use and those that have been set by agencies such as the USGS have been damaged or destroyed. This means a surveyor would have to traverse a long distance to locate one of these monuments.
New technology such as combined electronic distance meters and theodolites (commonly called Total Stations) have made conventional surveys much easier and less labor intensive. Conventional methods can also be mixed with newer technologies such as GPS to further reduce cost, improve accuracy and georeference data.
GPS has opened up a whole new way to easily collect georeferenced data. Unfortunately many users do not understand how GPS works or its limitations. A GPS unit purchased at a big super store for $100 does not have the same ability as one purchased from a surveying equipment vendor for $20,000. What a lot of people do not understand is that GPS equipment comes in various grades and prices depending on its capabilities.
There are three general grades of GPS equipment, each with different capabilities and levels of accuracy. The first is recreational grade. These units cost anywhere from $100 to $1,000 depending on the bells and whistles. They have a horizontal accuracy typically around plus of minus 10 meters. Some of the high end models can achieve a 2 meter accuracy. The next grade is GIS/Mapping. These units are capable of sub-meter horizontal accuracy plus they can also be used to record user defined attributes about the items being located. The highest grade of GPS equipment is survey grade. These units have a sub-centimeter horizontal accuracy in addition to having many of the attribute collection capability associated with the GIS grade.
Regardless of the grade these units share some common short comings. First their vertical accuracy is not always very good. In general a GPS units’ vertical accuracy is 1.5 to 2 times its horizontal accuracy. This means survey grade equipment is the only one that can record reliable elevation data. Second, since these units rely on satellites in the sky anything that blocks the sky can adversely impact the ability of GPS to record a location. This can include trees, buildings, bridges, and people. Weather and other atmospheric conditions such as sun spots can affect the reliability and accuracy of GPS data.
Station and Offset Surveys
This method is most commonly used for studies, utility relocation or preliminary roadway design. The Station and Offset method is a quick and easy method to locate objects as they relate to an existing road. This method consists of someone measuring a distance down the centerline or edge of pavement of a road normally starting from an intersection. Once they are approximately perpendicular to the road and the object to be located, they will record that distance as a station in the format of hundreds + tens of feet (i.e. 2+57.06 = 257.06 feet). Once they record the station, they then measure the offset from the station to the object being located recording not only the distance but also the side of the road the object is on. A measuring wheel or a tape is the most common tools used to take these measures. This is the least accurate method of surveying.
Other methods of data collection:
Just as we do in GIS, Engineers and Surveyors will often use existing data to start a project. This is especially for preliminary design phase of their projects. This existing data includes existing engineering drawings, survey plats, hardcopy maps, aerial photos or GIS data. This data is imported in to their current project using many of the same methods we use in GIS such as heads-up digitizing, raster to vector conversion, or doing a direct import. Because all of these methods make use of existing data, the accuracy of such information varies greatly. If you, the GIS professional, want to make use of this information you must learn as much as possible about how the drawing was created. The more information you have the easier it will be for you to decide how to integrate the drawing into your GIS.
As you can see not all CAD data is created equal no more so than all GIS data is equal. In order to effectively use CAD data in a GIS, you must know as much as possible about how the CAD data was created. It should be treated like any other piece of information you intend to add to your database. Keck & Wood, Inc. in partnership with Geospatial Training Services, LLC has released both an instructor led and e-learning class entitled Using AutoCAD data in ArcGIS that covers how to integrate AutoCAD data into a GIS database in much greater detail.
About the Author:
Tripp Corbin, a GIS Certified Professional and an ESRI Authorized Instructor, is the Vice President of GIS Services for Keck & Wood, Inc. and has over 16 years of Surveying and GIS related experience. As an ESRI Authorized Instructor, he is authorized to teach ESRI’s Introduction to ArcGIS I and II classes. In addition to the official ESRI classes, Tripp has provided customized training for ArcView 3.x, AutoCAD, and Autodesk Map.
Tripp is an active member of the GIS community. He is a member of several GIS and mapping related organizations including Georgia and Florida URISA, the Surveying & Mapping Society of Georgia, Georgia Association of Assessing Officials, and the Seven Hills Regional Users Group. Tripp is currently serving as Vice President of Georgia URISA.